When you edit an user via the Admin Interface you have an extra tab (User Kill & Ban) especially for that, where you can delete the user (optionally ban user via email/ip/user name) and automatically delete all posts by the user.
That doesn't actually delete the records from the database though, does it?
I ask because, and I should document this sometime because there are times when YAF says delete and it actually is deleted, but most of the time delete means the flags were changed or isDeleted is marked as true. I usually end up going through my database and find orphan records and manually delete them. It's always a little worrisome than I'm gonna break it though haha.