I don't think there are granular controls, but I honestly don't think it's as big a deal as you're describing. It'll get pretty obvious at an admin server log level what's happening. Brigading is brigading, whether it's bots or people. The admin of server1 (the victim) can then politely ask the admin of server2 (the attacker) to sort themselves out. Maybe describe the issue, and ask them to moderate their server better.
If that doesn't happen, then server1 would be completely justified in blocking server2. If server2 then starts brigading servers 3 through 10, they're going to get blocked there too. Then comes the angry man shouting at himself in a mirror step, where users on server2 turn to eachother. Eventually, everyone sane leaves.
~~Truth Social~~ Gab is a mastodon instance. This is what happened to them.
EDIT: Also, old-timers around here told me there have been multiple extreme alt-right Lemmy instances set up shop over the years of development. Every time they did, they got blocked by everyone else, bored with having nobody to shout at, then left.