From the Dota 2 website:

Today, we permanently banned 90,000 smurf accounts that have been active over the last few months. Smurf accounts are alternate accounts used by players to avoid playing at the correct MMR, to abandon games, to cheat, to grief, or to otherwise be toxic without consequence.

Additionally, we have traced every single one of these smurf accounts back to its main account. Going forward, a main account found associated with a smurf account could result in a wide range of punishments, from temporary adjustments to behavior scores to permanent account bans.