Delete the user account via the web UI. This won't delete their posts. To delete a user account you'll need admin privileges, and if you do it from the user's profile page you'll have to block the account first, otherwise the "delete" option will be unavailable. The User Manager plugin provides a more convenient way of deleting users without the blocking detour.
If you just want to disassociate individual posts from a user without deleting the user account entirely, I believe you have to do that in the database directly. Something like this:
UPDATE qa_posts SET userid = NULL WHERE postid = 12345;
Replace 12345 with the actual post ID. You'll probably also have to recalculate statistics after doing this (under Administration center → Stats).
