You can't. You also can't delete your account on this old relic of a forum.
Two of many things that needs to change here I feel.
Edit: Correction, I believe (unless I'm wrong again) that you can't delete a THREAD that you START if others already reply to it. You can delete replies I feel. Sorry for the inaccuracy of the post. I was tired after getting home from working a 12 hour shift that night.