Game developers now have the power to ban players on Steam. Until now banning has been handled by Valve and cheat systems like VAC and Punkbuster.
"Playing games should be fun," writes Valve in the announcement post. "In order to ensure the best possible online multiplayer experience, Valve allows developers to implement their own systems that detect and permanently ban any disruptive players, such as those using cheats."
Valve still applies the ban but it's the developer who decides who receives the ban hammer.
So, if you've been banned by a developer, you'll have to contact them instead of Valve to get the ban lifted.