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.
It's 2015 though, a simple 'delete account' link shouldn't really be too much to ask for.