It's very easy. Does the person take part in actively harassing, harming, destroying the lives of others with their actions and speech? Milo whatever his name is, does that pretty regularly. He incites his following to active suppression, and in some cases takes part in it himself. Thats no longer protected speech.
The intention remains completely different. MLK's intention was to create a new stage for colored people to voice their opinions in public discourse where none previously existed. Its present in his rhetoric. Milo and Spencer are working to tear down public discourse by suppressing voices across race, gender, sexuality, and religion. Those topics are also present in their rhetoric.
If you actually listen to what they have to say, they're not doing anything to "tear down public discourse," they're actually trying to introduce more (highly controversial) ideas into that discourse. It is actually the liberal establishment that is acting to shut them down, either through tactics like no-platforming or otherwise disrupting their speaking engagements.
A peaceful ethnic cleansing is a new and interesting idea that deserves discussion? Really? Me having to defend why I deserve to not only have a seat at the table, but live in the country I was born in? Normalizing behavior that actively works to silence my voice is absurd, and counter to freedom of speech. Milo Yiann-whatever uses his platform to do shit like OPENLY mock a transgender kid on stage? To dox and verbally harass people from every stage of life? I've actually heard the dude speak in public, and was promptly told by a few people in the crowd I should go back to my own country and I'm from fucking Puerto Rico a fucking US territory. Liberals at the talk were openly mocked when asking questions, and made fun of by the surrounding crowds. Wtf is that? That's freedom of speech? I actually left that talk because I physically didn't feel safe there. That's not silencing my freedom of speech? By normalizing contempt/hatred/etc. for others you are actively working to silence those groups. Sorry I don't find that defensible its very easy to see where the line is drawn for me.