Allow it. Let the racists be racist. Let them expose themselves so we all know who to ridicule and ignore. Not only has this wacky notion of free speech worked well for a long time, society has grown far less racist despite no official policing of speech.
The insidiousness that comes from controlling speech is far greater than the insidiousness of a small group of assholes who say mean things. The short-sightedness of many continues to astonish me.