You can't stop them from posting on your threads, Putting them on you ignore list only hides their posts,, http://www.rcuniverse.com/forum/prof...?do=ignorelist

but you can block them from posting on your ads,, go to the bottom of your "my market" page, you'll see the area to add to the block list http://www.rcuniverse.com/market/mymarket-selling.cfm

good luck