I keep seeing people ask for this. There are basically only two ways, neither of which are terribly easy unless you are willing to switch to a Lemmy alternative and then it can be a breeze with just a couple of button clicks.
First, note that on base Lemmy, it basically cannot be done, short of either spinning up your own instance or trying to do some advanced programming with spamblock filtering rules (that is likely to mess up the pages in some way). There is a related feature though - in User -> Settings -> Blocks -> scroll waaay down -> Block instance - except that unlike blocking a community or a user, this does not actually "block an instance", and instead merely (& misleadingly?) hides the communities on those instances. You will still see comments from those users, they can still downvote you, and ping your notifications, etc.
About the only thing the above approach offers beyond blocking those communities individually is that if ever new communities were to be made from those instances, they would be automatically hidden from your account. So not all that helpful imho.
(1) Use an App
I have heard that the Sync and Connect apps (+ maybe others?) offer this, as well as a plethora of other features. Note that Voyager does not work for this - it is the same type of blocking as mentioned above.
Check them out? If anyone wants to supplement this section, please submit a post to this community to help people who want to know! (and/or at least add it in the comments here)
(2) Lemmy Alternatives
What I do use is PieFed.social, which in addition to this feature also offers several other advancements not currently available in Lemmy such as Categories of Communities that makes finding additional content a breeze (though overall it is not as feature-rich or easy to use as base Lemmy; and yet its choice to use Python rather than Rust should help it to catch up extremely quickly, plus the admins are extraordinarily responsive to deal with any issues).
To block all users from a PieFed instance, the easiest way is to start from a user on that instance, click their account, then click More -> Block everyone from [instance_name]. Or you could go to a page with the instance name in the url, like https://piefed.social/instance/lemmy.ml and just click "Block everyone from [instance_name]" there.
PieFed also offers additional opportunities in-between blocking trolls vs. not doing so: accounts that meet certain criteria levels will have icons placed next to the account name, so that you can still see their content (rather than have it automatically removed) but not have to spend as much time parsing it as you would something that is more likely to have been offered in good faith.
Mbin likewise offers Categories, and cross-connection with Mastodon. Overall I find that whole style confusing - e.g. "communities" become "magazines", downvotes become "reduces", upvotes are both "favorites" and also upvotes exist too that are entirely separate from that, plus you can see who offers favorites, but only from other Mbin/Kbin users and you cannot see the same for reduces. ~~Though if you want Mastodon integration with Lemmy in one account, this is definitely the way to go (b/c it's the only one that does both:-)~~. From @nictophilia@fedia.io:
It's not anywhere in the settings at all, lol. Like a hidden option. You have to go to the url
https://fedia.io/d/[instance_domain_name], like https://fedia.io/d/lemmy.ml. Then it will give you the ability to block, and that block will be reflected in your settings page.
Edit: according to @DarkThoughts@fedia.io, this does not actually work:-(.
Either of these alternatives should make you quite happy with the result!:-)
(3) Honorable mention: relying upon an instance admin
As a normal user, not an admin yourself, you cannot implement a custom block of users from any specified instance. However, you can either ask your current admins to implement such a block for you (would need the support of the entire community on that instance ofc), or move your account to one that has already done so?
The only instances I've ever heard of that block the big-3 (lemmygrad.ml, hexbear.net, and lemmy.ml) are:
- lemmy.cafe - has very welcoming messages, including a link guiding new users to this community!:-)
- Tesseract on dubvee.org - extremely impressive, if not for everyone, but definitely worth a look
- quokk.au
The caveat to all of these is that each is a single-admin instance. Those of us who recall the story of e.g. Kbin.social (or dmv.social or so many others) know how worrisome that can be in that it could vanish overnight with little to no warning. Then again, unlike Kbin.social, they seem quite healthy for now - definitely worth at least taking a look?
I mean, if they choose that for themselves then sure. Or perhaps a top 20 instance will choose to defederate from lemmy.ml (and some also from hexbear.net too). There are lots of things that could theoretically be done - e.g. sh.itjust.works is currently the #4 instance wrt monthly active users, so if those admins were to implement such a block, and also make sure that it affected the view when looking at content as a guest without an account, that would be an even more major solution. Though some people may avoid it purely for the name alone:-).
And yet that applies to so many of them - Lemmynsfw.com, lemmy.ca (implies it being regional, which tbf... it is!:-), feddit.org, lemmy.dbzer0.com (not everyone, especially mainstream, may be comfortable with anarchy), lemmy.blahaj.zone (I for one find it very welcoming but not everyone is gender diverse or comfortable with that I suppose), programming.dev (also other issues there with their database I guess, plus not everyone is into programming), discuss.tchncs.de (wait that's... not even Lemmy, that's PixelFed - why is that in the Lemmy list at https://lemmy.fediverse.observer/list ?), and so on and so forth. The best from a "URL name" standpoint with high MAUs may be lemmy.zip, but it is focused on gaming, which not everyone is into, and they remain federated not only with hexbear but also lemmygrad as well, so don't seem amenable to want to defederate from lemmy.ml on top of that.
Nothing is perfectly suited for the goal of becoming the next Lemmy.World as a general-purpose instance that could absorb Reddit refugees, but split the traffic at least moderately so that it does not all currently reside on one instance. And even if some solution could be found - like if lemmy.cafe were to decide to take on a new admin - could they even handle a large influx of thousands of users, and continuously maintain that down the road? That particular admin seems on the ball (not that I know much about such things, and as you pointed out previously, running a beta software version may actually cause issues with federating with others instances), but that has indeed been a limiter for many other instances like programming.dev and startrek.website. If they want to and they can handle it, then that's great, but it's up to them to make those calls.
Which is why that conversation with the admin of sh.itjust.works was so exiting to me - they are already set up to handle that kind of traffic, they already are the #4 highest-traffic instance, and they have already defederated from hexbear, plus have expressed sympathy for doing at least something about showing Lemmy.ml content to their own users. Yeah the name may turn people away, but what other options are there? Though it's up to them if they want to aim for such, just like it would be up to Lemmy.world to decide similarly how to handle content coming from Lemmy.ml.
In the meantime, I wanted people to know how they could take matters into their own hands without having to spin up their own instance. None of which is terribly easy unless they are okay to switch to using a limited set of apps or PieFed or Mbin. There are also many other benefits for doing the latter as well, including to further become less dependent upon pro-authoritarian developers. At the end of the day, it is what it is.