Deleting accounts is a tricky thing, and as a rule we just don't do it. The problem is, either we dig through at the database level and yank anything with your user ID - posts, PMs, wall messages, etc, etc, or we leave them, and they become not linked to a user account. The first option is incredibly tedious. The second tends to corrupt the database.
So, essentially once you've posted, your information becomes part of the internet, and will be there in some form likely forever.