That happens all the time with spam mail servers which is exactly the same problem beehaw was having. If you set up an email service that hosts spam bots and do nothing about it then the other email services will start blocking it.

Beehaw was being spamed with dick pics and other shit that was actively malicious. They could ban those profiles but then those users just went and made new profiles on lemmy.world or one of the other instances with open signup. Right now Lemmy has limited moderation tools so the only way to perminantly "ban" those users is to defederate with the instances that keep hosting them. They plan to refederate with those instances once better tools are in place to perminantly block users like that.