In a new post titled "Cheaters Will Never Be Welcome in Dota", Valve mentioned how they managed to get a whole lot of cheaters to show themselves and then gave them a swift boot to the buttocks.
Valve mentioned how they managed to ban 40,000 cheaters that were using third-party software over the last few weeks. How though? It's actually quite clever! They created a honeypot to trap people, and similar to a way you used to be able to fight some spam bots on website registration forms.
This third-party cheating software was able to show people things they weren't supposed to see during normal play, giving them an unfair advantage. So Valve added a bit that no one should see, which this cheating software showed people and so Valve could then have "extremely high confidence that every ban was well-deserved".
Valve make it clear that while the initial ban wave was "particularly large", it's just all part of their ongoing effort to remove cheaters from Dota 2, which is usually done in secret but they wanted a public example of their effort.
Good job Valve.
Dota 2 is free to play on Steam.