Why would it not be okay to just ban people from your service "because you feel like it"? Given that you usually don't have any info on them which can be used as the basis of discrimination (and thereby sued over) what's left to worry about?
I would call that enforcement of a term of the service.
When I said arbitrary terms of service can't be enforced, I meant that the site owner won't have good luck trying to obligate users to do things (their typical response to a problem with the user will be limited to withholding access from the service). I wasn't very clear.
