Actually, thanks to technology, it is very easy to enforce a ban. There are of course camera at the turnstiles and there is software that can very quickly identify a person and compare it to lists of people (photos). Then the turnstile can deny them or security can be dispatched as needed. This is my guess as to how they would accomplish this. Also, given that the number of banned guests is probably very low, it is very likely that security can simply remember the faces and then double check if they think they see one of them.