It only leaves the placeholder behind if there are replies to the post. Otherwise you'd break threaded mode. <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" /> So delete the replies to that particular post first, then it will delete without a trace.
There is a mod at
www.threadsdev.com to automatically reassign any replies to a post you delete to the main post in your thread. This messes up the logical order of threaded view, but allows you to delete without a trace. <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" />