You can. All you have to do is go to the post, click edit, then click delete. Once they're so old, you wont be able to... not sure how long that is though.
EDIT: you said thread while I was typing. I'm pretty sure you can still delete a thread as long as nobody has posted in it the same way... You may still have to have a mod or admin delete it though
I know for sure that once it has been posted on by somebody else, you cannot delete it... as to whether you can before somebody else posts, I am not sure. I'm sure Mike or somebody else will be along shortly to give you a definite answer though
You can. All you have to do is go to the post, click edit, then click delete. Once they're so old, you wont be able to... not sure how long that is though.
EDIT: you said thread while I was typing. I'm pretty sure you can still delete a thread as long as nobody has posted in it the same way... You may still have to have a mod or admin delete it though