I don't believe there is an objectively right thing to do. Inherent in the right to free speech is the right to allow dangerous speech to spread - speech isn't value neutral. It balances against the right of association - the right to speak is not de facto a right to be heard on all platforms. Choosing where to draw the line is a matter of personal preference.

Personally, I believe even reprehensible people have the right to speak, congregate and spread their ideas, but also that society has a moral duty to prevent the spread of harmful speech even in the name of free speech (see the paradox of tolerance,) and they aren't entitled to an arbitrarily large platform. Of course such a position is fraught with the potential for abuse, since the definitions for "harmful" and "useful" speech are arbitrary (speech against slavery was considered harmful at one point of course.) but I still think this position is preferable to the alternatives.