Last Thursday was a banner moment for honest gamers, and a ban hammer moment for tens of thousands of rotten cheaters. That is because Valve lifted its mighty VAC (Valve Anti-Cheat) ban hammer and with one mighty blow, it obliterated more than 40,000 Steam gamers who had been caught cheating. It was the the largest single-day ban in the history

Source: Hot Hardware – Valve Swings Anti-Cheat Banhammer Smashing 40000 Steam Accounts Following Summer Sale