The thing is, when that happens, the mods/admins of the trolls new instance ban them there, without the original admin having to do anything.

Abandoned instances, or instances that simply don't moderate, get defederated, so it's quite manageable.

The only case where it's not that clear cut, is where the troll is a big issue for one admin, but not another. Take trans issues for example. I have a zero tolerance policy on transphobia on this instance, but not all lemmy admins are as aware of the dogwhistles as I am, so I will block some users that other admins won't. It's not ideal, but it's manageable, because I can stop their brand of transphobia from reaching my instance even if they're not banned by the remote admin. And if that pushes the troll to create another account elsewhere to get through the instance ban, then that becomes harassment, and the other admins will act, even if they wouldn't before.