There is no way to do that in Q2A currently. As an admin, you can delete a user (which doesn't delete all their posts AFAIK). And you can hide all posts by a user. So the user would need to ask an admin to delete their account and posts.
Depending on your T&C you may not want to hide/delete their posts, as it would cause gaps in the site and you could lose good content.
The only solution for users would be to simply change their username and delete any profile info, then their account is as good as deleted.