I seem to recall people telling me a jerk would harass them and not leave their game, and if they were booted, they would come right back. That shouldn't be possible if you ignore them.
To ignore someone, you click on their name in the server player list and click the Ignore button.
Note that the HOST of the game controls who can join a game. So if you have someone ignored and you aren't hosting the game, that person may be able to join the game.
If people are still able to harass you (and keep joining your game) after you put them on ignore, please let me know the details and i will investigate.