DELETE from users WHERE user is spammer

How do you distinguish spammers from normal users in a phpBB2 board using SQL? Not easy. I just did a simple

DELETE FROM phpbb_usersWHERE user_id != -1AND user_posts = 0AND user_website <> "" AND user_new_privmsg = 0AND user_unread_privmsg = 0AND user_last_privmsg = 0

causing the deletion of 5000 of all 7000 user accounts of the Irrlicht Engine forum. I guess I caught most of the spammers. But some poor normal users as well, for sure. I hope they don't get too angry.