Yeah, no, you can't physically prevent that, not without a perfect panopticon police state.
If you allow intolerance to spread, it WILL, without fail, hurt people. Sometime, immense number of people. This is not theoretical. It happens, again and again, and the only way to prevent it is to oppose intolerance at every stage.
Your abstract ideology of absolutely free speech is not in any way worth the immense pain and suffering that will, inevitably, follow.
On the contrary - you can't prevent intolerance from spreading without a perfect Panopticon police state. That's precisely why Europe is failing at it so badly, despite all their hate speech and extremism laws. Organizations like AfD can dog-whistle in public to avoid crossing legal limits, while still fundamentally communicating the same ideas. You could crack down on that if you had a pervasive surveillance state monitoring all private communication, but they aren't willing to go there.
On the other hand, policing actions is much easier, because the more consequential ones are also the more prominent - you don't need a surveillance state to deal with them, you just need a reaction force.
If you allow intolerance to spread, it WILL, without fail, hurt people. Sometime, immense number of people. This is not theoretical. It happens, again and again, and the only way to prevent it is to oppose intolerance at every stage.
Your abstract ideology of absolutely free speech is not in any way worth the immense pain and suffering that will, inevitably, follow.