You can ask for deregistering your account. In addition, all threads started by yourself can be deleted by yourself with the exception of Consumers Corner and Buy & Sell. However, if these threads are of common interest (e.g. in Newbies Corner) they might be kept, still.
Postings in other people's threads remain. You are still responsible for what you posted and threads will become meaningless of postings get removed just because a user leaves the forum. Also, bear in mind that Google agents sieve through the forum daily, creating copy of the content in Google cache. The Internet does not forget (easily).