There is a way for administrators to do this, but not for regular members. Ordinarily, old posts are kept as possibly informative for people searching for answers to questions already answered. There is a wealth of information available here.
The staff can delete posts, but we normally don't. If there is a post or thread violating the rules or that you feel needs to be deleted, use the report button for that post and we will review.
But if a post or thread doesn't violate any rules, we won't normally delete them.