> How do you stop that?

Better minds than mine have debated the paradox of tolerance (a name only given in the 1950s, but a concept recognized for longer).

Also, not tolerating something doesn’t mean forcefully censoring it. Norms and conventions largely keep e.g. frowned-upon slurs out of common usage. The person getting told to shut up for using the N- word isn’t being censored by the state, but they are being encouraged to engage respectfully.

In practice, it seems that whenever someone is told to shut up for using the N- word, they react by saying it many more times. So it doesn't work very well.