They're not denying you access for no reason, though. Their reason is that they banned you, which they do notify you when that happens. They don't need more of a reason than that.

A breach of contract would be a case where you can't log in, are given no reason why you can't log in, are unable to get any sort of reason from SE when you try to contact them, and yet they continue to charge you. Even then, good luck getting any kind of legal action taken against them.