Valve has made it easier for developers with games on Steam to ban players. Under a newly deployed system, developers have the full authority to ban players, ultimately taking the burden off Steam. The new system gives developers oversight and responsibility to ban cheating or disruptive players as they see fit.
This also means that any player appeals of bans will have to be directed at the people who make the game and not Steam/Valve.
Here's more from the Steam community announcement: