Actually, it is a trademark issue, as the owners of the Southpark trademark could take the domain away from you if they wanted to. They could do it at any time because the trademark is in the domain name.I don't think trademark is an issue since its only a fansite domain.
Whether they actually take the domain away from you is another story, though. As long as they're happy with the forums and it's not negative towards Southpark then they probably are unlikely to want the domain. Unless, of course, they want their own forums.