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.
The problem with delete accounts is it could cause gap's in threads. And depending on account be a real mess. The main problem is scammers.
Imagine if a high level scam account was able to delete it all. One second account, and threads, etc all gone. This would be a mess for those effected by the scam. Sadly there are scams that happened I almost was taken in Terrahash scam. It's these accounts that might even have legal actions against them that we would not want deleted.
I think delete would be a mess. It would have to be done on case by case so ties up staff or moderates to do.
If you truly want to delete account you could delete every post, switch email to a dummy one, change password to something you would never use, and leave it be. If reason of delete is worrying about compromise this I would guess would get rid of a lot of the data you don't want out there.