Sometimes it’s more important to stick to your morals than to get that new guild member. Not to mention that assuming your server rules were catch-all “Don’t be a buttface” rules, this will come back to bite the other guys in the butt sooner or later when their alliance becomes a nest of vipers because they have zero consequences for their behavior.

like with any group thing, allowing toxic behavior will eventually ruin the group. Let the other people know why you booted the person. Good chance the others got a different story than you have so the person could join. Also, removing toxicity from your own group (game, home, anything) will make your life better even if the other group does manage to get ahead in some way, it will most likely cost them stressed out people and possible mutiny down the road... things I have seen in my life playing group games.

With any luck, this will turn in a soccer match where the person will be booted from one guild to the next until no guild will taker him/her anymore, or until he/she agrees to the rules.

Well, I mean, as a free agent they can obviously join whatever allegiance they want, no? Or did you expect them to join a convent in mourning?

happens all the time in my game. also let's not start on the greifers