It's kind of amazing how similar the boundary-pushing fascists engage in matches what children do when you ask them to stop doing something and they have a desperate need to prove that the thing they were doing was actually OK. They force you to choose between letting further transgressions go or feeling like an asshole, and then once you decide to give up and let it go, they have a new boundary they can start from.

The only response that works is to already have a good idea what the boundary is, and stick to that boundary, calmly and persistently. If they keep crossing it, then you say, "You can't be in here if you can't respect the boundaries I've set" or something along those lines.