Yes, but I'm not so much interested (right now) in what are the optimal rules to impose on people; I'm asking what is the right thing to do, which is a subtly different question. Your argument that eviction leads to problems in other places is clearly true. Analogously, it would be a very bad idea to impose a 80% marginal tax rate on top earners to fund the Against Malaria Foundation, because most of them would work less and there would be huge deadweight loss. However, Peter Singer and people like that argue persuasively that very wealthy people should as a matter of principle voluntarily give a high percentage of their income to efficient charity. And this causes no deadweight loss if they make sure to work as much as before.

Similarly, if there are creeps in your group, don't you wish they would just leave, and not try to infiltrate another innocent group? Then that is what they should do.