It's really impossible to tell what the user wants to do, which is why you need to be very restrictive in the free hosting world if the signup is just out in the open.
Ex, worried about spam being used in say, PHP on the hosting, disable the mail(); function in PHP. Make spamming not possible. Worried of PHP proxies? Disable fsockopen(); in PHP.
Take measures to block people from doing this kind of thing in the first place, its quite common in free hosting and its hard to figure out whats abuse and whats not.