[AOsborn]

Dota 2 addresses this specifically with a time-sink. A minimum number of games is required to play competitively, which increases the cost related to "new accounts at will".

In addition, even for non-competitive games, players who obviously troll and are flagged by the community are pushed to a separate pool with other trolls.

The system isn't perfect but appears to work fairly well for new players.

[lijason]

League has the same deal. Also, in League, you either have to sink more time or pay real money to play the champions that you like, so that further increases the cost of creating new accounts.

[RiotBoatStuff]

You can buy champions with IP (in game currency) and not RP (real money). The only thing you can only buy with cash is cosmetics. All runes/champions can be obtained for free.

[Razengan]

GP did say "either sink more time OR pay real money, to play the champions that you like."