Be very very very careful with this! Just imagine if you you are user 42 and the spammer is user 52, and you accidentally deleted user 42! Opps!
What might be better is this, especially if this is the first time doing it:
Instead of deleting them, perform an update SQL query that makes all of his posts hidden. That way if you screw up, they are still there. Also, its good the save the evidence in case of a dispute or you have multiple moderators / admins who need to know about the abuse.