And those instances certainly does that. Take a look at the meta thread in lemm.ee proposing defederation from hexbear, where hexbear shows up in brigade proving the point of the request and causing the thread to be locked by brigading and shitposting.

Same shit went on in other instances when people proposed defederation from hexbear, hexbear users flooding the threads and brigading the hell out of them, which is problematic systematic behaviour on its own, but especially so when the victims are smaller instances with only few users.