Maybe we should have some spam fighters? Like, fight fire with fire? So whoever created the topic we just have 100 people send them 5-10PMs a day saying "lock your useless thread *link* or the spam to your inbox will continue.
Fighting spam by producing PM spam is actually a very horrible way of trying to accomplish this. Some of those users also may not want to lock their threads for the obvious reasons (they are either spamming themselves or facilitating spam).