We can't ban communities, because they aren't people and don't do actions. iirc there is a proposalto "temp-block" sites in the same way admins can "temp-ban" users, but we haven't been working on it.

Lemmy's bot problems used to be muuuch worse than they are now, and I encourage most instances to use the registration application method of signups, which has been time-tested by older forum software, and which we've found to work the best for blocking bots.

As far as admins notifying others, we don't want to reinvent the wheel by creating a chatroom in lemmy, so we recommend admins / mods use matrix or something else to communicate with each other about these things.

As an admin you can remove a remote community, then it shouldnt federate anymore. Though admittedly this isnt very intuitive.