I don’t think there is a way native to Lemmy for a mod to do that. You would have to talk to the instance admin you are on to defederate them at the moment. This might change in the future.
What you might be able to do is see if anyone has made a mod bot yet. Then you could configure the bot to ban anyone who posts from instances you don’t like. So reactive bans instead of preemptive.
I’ve seen the first couple bots pop up so I would expect to see a mod bot pretty soon.
It’s really location dependent. Some places you’ll see the birds in seconds and others never. You’ll need to know the climate, local weather, soil conditions, what else is growing in the area, time of year, etc. to get an accurate answer.