While it might sound scary that more than 40,000 Dota 2 players got their accounts banned, but in all honesty, it's actually great. They were banned not for hacking or anything, though and instead were banned for abusing matchmaking - aka smurfing.
As a MOBA player, I can assure you that nothing is more painful than to be destroyed by a low-level player when you just the handle of the game. With this, many new players rage or simply quit the game, as the soft start becomes a struggle for surviving. My first encounter with a smurf resulted in a nice score of 0/22 KDA. Valve's opinion on smurfing was pretty clear - they don’t like it at all:
We are making our smurf detection system more sensitive in this update. This change will...