It's simple. Delete all of your posts and messages. Change your password to a random one which you aren't going to remember. Effectively, deleting your account. Not that it makes much difference if we are assuming that the hacker has a full dump of the site. As he will have that information regardless if you delete your account or not.
Have you ever tried to delete posts on this forum? It takes forever for each one. If you have hundreds or thousands of posts, it would be an impossible task. You'd need to write a script to do it or something.
It takes a few seconds for each post. besides, you don't have thousands of posts. If you did you would be unlikely to want to delete all of them. Allowing people to delete content with ease wouldn't benefit the forum. There would be a lot of information missing, malicious users would be abusing it and theymos would likely have to restore a lot of content deleted by malicious users.