About a year ago, Valve blocked several players from participating in their sponsored tournaments when the players were believed to be match fixing. This is the practice of arranging outcomes in events and tournaments. This is often accompanied by betting on the pre-arranged winners, but it could also be used to shift around positions in seed brackets by having one or more member intentionally lose winnable games. This is bad all-around, but can even be illegal (due to the implications of fraud and so forth).
Since then, the game developer has reviewed their earlier decision, and they decided to make it permanent. They did not state how many players were involved, although PC Gamer knows of 21. These individuals will never be allowed to compete at any Valve-sponsored tournaments, and other organizers will be able to extend those bans to their events, too.
A similar incident happened in the Korean StarCraft II scene. In that situation, a dozen individuals were arrested and detained by Korean law enforcement, charged for betting (or enabling third-parties to bet) on predetermined outcomes. This has been an ongoing problem.
