Child porn, illegal gun sales, illegal drug or paraphernalia, Con games, fraud or any of the like.
Sites doing that will have some policing authority wanting copies of your site and all other information we have on the account - within days (if not hours) of when the site first operates illegally. Again, we don't need the Hassle and the server doesn't need a reputation of needing to be watched. Immediate account termination without refund, once a complaint is verified.
Remember, the sites are frequently backed up and retained for restoration in case of a unlikely problem. That means they're also subject to being handed over in the case of a legitimate subpoena.
While were nit a convent of Nuns, we really only want good neighbors on our server.