Just ignoring these players is not enough, just because you can no longer see what they type doesn't stop them from being a complete nuisance to players trying to enjoy the game.
In other games I have played, when you ignore someone it actually makes that Players invisible to each other.
This is the most effective way of stopping the harassment of players as defined in the Terms of Service under "User Conduct" sections
(f) "Stalk," threaten or otherwise harass or cause discomfort to another;
(n) Take any action that disrupts the Service or that negatively affects or may prohibit other users from enjoying the Website, the Games or any other aspect of the Service.
It seems that complaints made are not stopping these players from being a constant violation of those rules.