Unlimited tolerance must not be extended to the aggressively intolerant, because this will destroy the unlimited tolerance. Central philosophical principle of free society. Karl Popper.

Popper was specifically referring to levels of intolerance that cause people to move from the world of discussion to the world of physical force.

That is, per Popper, tolerating people who will physically harm you as part of the discussion means a discussion cannot be meaningfully held at all.

But people really like to stretch this to whatever edge-case meaning of "intolerant" would be convenient to them at the time...

Well, the law certainly represents physical force.

Yes. Certain opinions you cannot allow to exist if you want your democracy to continue to function.

Incorrect. There is no such act as "not allowed to exist". Instead, rather than attempt to bury what is spontaneously manifest, we ought lift these bolstered options up and publicly demonstrate to all how they are torn asunder.

Not allowed to exist is an unfortunate formulation. Marvin above said it better.