You can't. You can delete your own posts and threads that have not been replied to, but you can't otherwise. It's not good to delete your posts anyway, if someone with a similar problem to yours finds your thread then they can solve their problem.
If you are in desperate need to do this you can edit your recent posts and put something like ellipsis in them. Of course, it is considered rude in polite society, but luckily for you this is a programmers forum ;) (programmers endure rudeness longer)