Yeah, unfortunately phpBB is limited in that unlike vBulletin you can't delete your own threads if someone has replied to them. The only way around this is to give everyone moderation powers, which would mean anyone could delete anyone's threads, not exactly the most sensible solution.
Another way around it is to modify the code for the board manually, but forum-motion is a rather basic forum solution and you don't have full control over the forum, so unfortunately we can't do that.
So yeah, the solution is to PM one of the mods or admins here and we'll remove whatever needs to be removed.