Every voice must be allowed to say their piece or you are not tolerant.
They can say the most heinous things, but until they threaten or hurt someone silencing them may legitimize them, or maybe you're wrong but your own prejudices and predilections blind you to the fact.
Do you tolerate intolerance? Do you tolerate somebody being an asshole, or being a racist? How far does your tolerance go? This always reminds me of the old canard that we have to tolerate old people being awful because they're from "a different time."
You don't stop racism. Telling someone to stop then shaming them doesn't make them stop being racist it makes them resent the other races it makes them resent them more. The way to stop it is expose them to good people of other races and hope they chang there mind.
They can say the most heinous things, but until they threaten or hurt someone silencing them may legitimize them, or maybe you're wrong but your own prejudices and predilections blind you to the fact.