I wish Valve had some sort of monetary incentive to fix this. If this happened to the cash cow that is Counter Strike this would have been fixed already. It's sad to see this going on especially with the resurgence TF2 has been having lately.
The fact that they're letting this shit happen to one of their own communities is extremely disappointing. They should take it personally that a group of people are using their game as a tool to harass people, but they don't give a fuck.
I have lost so much respect for Valve over their handling of TF2. I've gone from a fanboy to a hater.
That's precisely the problem; they continue to support the game, in the state that it's in. Honestly, an appropriate move would be for them to officially discontinue support for the game altogether (even if that means not having a new IP to replace it with). This means turning off their official servers, and removing the ability to buy/sell inventory items on the Steam Marketplace.
Right now, Valve allows people to sell items for real money (which they take a cut from) in a game that is overrun by bots. They need to either fix the bot problem, or stop taking people's money.
Up until they release the intellectual property into the public domain, alongside the source code and hosting tools.
Corporations are not to be trusted to maintain and preserve our culture. They have enthusiasticly proven again and again they are irresponsible and cannot handle this task.