I think that the thread creator should be allowed to delete replier's posts in all forums except for Misc. and Art, and in those two you would need a moderator of any forum or administrator's permission. This would reduce spam in serious threads or in general unwanted replies. The Misc. and Art rules would be there to protect replier's opinions on miscellaneous subjects and in Art to protect a responder's opinion on artwork (i.e., deleting a post from a responder that dislikes the work.)