I'm banning with BE of course. By steam account, I'm reffering to their Arma 3 game (sry if it was not clear). But each time they have another copy of the game and come back.
The "need to buy another copy" is quite naive I'm afraid, there are ways to obtain for free Arma 3, and I think they know it (or they've spent a LOT already).
I will take a try on the IP thing, but they will know how to overcome that (it's easy).
No more ideas ?