Apparently you can not delete a post after someone has replied to it (I posted from the Quick Reply box, which the forum assumes is a reply to the last post in a topic).
At least with a new topic you used to be able to delete it after a reply, but that would just replace the body with 'post deleted'. The only times I've tried to delete a reply was when someone replied before me to answer a question (while I was typing, or between me checking for new posts and posting), so I deleted them before anyone could reply again.
