This ability was removed from member accounts because of abuse. When a member deletes a thread all posts in that thread also vanish. Is this fair to the people who put time and thought into replying? No. Since 'purging' is a regular feature of our community when people suddenly decide to leave, we felt it best to not allow a single member to perhaps have the abiltity to kill possibly kill thousands of posts by others.
If you start a thread then you can ask a mod to kill it.
Also: Actions bear responsibility. Sometimes when you post it shouldn't be easy to vanish. You said it, you should accept the results from having said it.
Myriads