Using an SQL Query would be complicated and break lots of stuff, as the "delete" process updated board totals and accounts for any replies to posts that are made.
If you delete his posts, and there are replies to them... the thread will be "broke" when you try to look at it.
But - from the Admin menu, under Post Management, there is an option to delete all posts from a user. That offers 2 options as to how you deal with replies to a deleted post.
