well... if by "close" you mean delete that is only available to admins.
If you mean to have it Closed/locked, i would recommend simply editing out your first post and putting "thread closed" so that anyone visiting knows it is a done deal and a mod will eventually close the thread to additional posts/edits. If it is an auction thread and you cannot edit the first post, a final post stating that is closed should suffice. IF you, for some reason, REALLY need/want a thread closed, simply send a PM to the moderator associated with your particular thread's forum (look at bottom right) and ask them to lock the thread. Cheers, SP