The solution: Do not respond to posts that are placed in the wrong category, or simply respond with the proper category only (i.e. 'Ski Equipment.' or 'Non-Ski Gabber'). Once one person has replied with the proper category, everyone else should just let the thread make it's way to the bottom of the list by not adding to the thread.
If everyone does this, it will work. It's the most pollite, and effective way to fix the problem I think.