Unfortunately, in any online game (especially a free one) you're going to get many forms of abusive behaviour. One such problem in Dota 2 is 'smurf' accounts and Valve are attempting to deal with it.
A smurf account is where an experienced player will make another account, to then stomp all over less experienced players and ruin their fun. It is a nuisance, it can completely ruin games and no doubt turns away plenty of potential fans. Valve are aware and they've blogged a few times recently about their plans to improve Dota 2's matchmaking and detection systems a lot of which is already live.
During this, they announced on Twitter yesterday that they have "banned over 40,000 accounts for players who were found abusing matchmaking" which will show "as game bans in Steam as well as being matchmaking bans in Dota 2". Additionally, on the official Dota 2 blog they announced that their 'smurf detection system' was being made more sensitive to go after more accounts. With all this, we could be seeing more regular ban waves coming in.
Policing online games as popular as Dota 2 is always going to be an uphill battle, at least it seems Valve are steadily trying to improve it for the majority of players.
You can play Dota 2 free on Steam.