Well I don't know if you can remove threads... you could edit the content, but if you have passed that timing and can't, PM the mods (their usernames are in green - namely EveWasFramed, Minus, Sci-Fi) to help you out on this.
A moderator, but we usually don't do that. PM one of us anyway and let us know which thread (with the link to the thread) and your reason why you want it deleted. It has to be a very good valid reason, not "just because I want it removed" or "I regret making it now" etc. Deleting threads affects more than just one person, and the thread is never truly gone either. There are backup databases out there that store forum information.