It should happen before the match, not every round. Like a DOTA game. Each team captain gets 5 votes or however many is decided upon, and they take turns choosing which race is banned for the whole MAP.

How sweet would it be?

Team 1: Fagabond
Team 2: Spiderfag
Team 1: Human
Team 2: Molecule
etc...

Hahahaha! It would be funny having only knife races available.