Ya can't. If you're quick enough you can edit the text and get rid of everything you wrote (except the title), but the ability to do that ends pretty quickly. Don't know the exact amount of time.
In the early days of talkbass we had the ability to delete our threads, and it seems I'm the only one here who liked that much more than the current inability to do so. I, for one, was a lot more free about things that I posted. And I thiink people in general would take some chances they might not now, and be a bit more candid. There are people here however who I think get their feelings hurt if anyone other than a moderator decides to obliterate something they contributed to a thread. Again, I have no problem with someone deleting a thread I contributed to, if they were the creator of the thread. I believe they should have the power to take it down... but whatever. It seems like it ain't gonna change.
So uh... ya can't delete your threads.