Valve says this is just one step in the ongoing fight against cheating.
A metal hammer with a rubber grip laying on a wooden table.

It's not too often that Valve comes out swinging their big ban hammer, but yesterday was one such occasion. On the official Dota 2 blog, Valve revealed that they have recently permanently banned over 40,000 accounts that were caught using third-party software to cheat. Valve used the cheaters' access to information "that wasn't visible during normal gameplay" to confirm their suspicions and dish out some bans.

Valve says that they released a patch once they understood how these cheats were working. Here is what Valve says they did to catch the cheaters in the act:

This patch created a honeypot: a section of data inside the game client that would never be read during normal gameplay, but that could be read by these exploits. Each of the accounts banned today read from this "secret" area in the client, giving us extremely high confidence that every ban was well-deserved.
Valve says that these bans and the patch are just "the latest action in an ongoing campaign" to combat against cheating. Valve makes it clear that if you are caught "running any application that reads data from the Dota client as you're playing games, your account can be permanently banned from playing Dota." Valve goes on to note that "this includes professional players, who will be banned from all Valve competitive events."

The Dota 2 developer says that fixing underlying issues that make cheats like these possible is a priority for the team.