The constitution gives you the right to pursue happiness, which a nazi rally is actively campaigning against for a large group of people.
Maybe, however you then have to try to draw a line and that gets insanely hard to do. To that end as reprehensible and abhorrent as their ideas may be, and worse still to my sensibilities lacking any real foundation in logic, until they cross the line of specific directions or incitement such that someone is likely to get hurt as a direct consequence (the classic example being something like if they were screaming for the killing of all purple eyed people and directed them to do that where they were at -- they may not know have known the name or even that one was around but as a direct and foreseeable consequence...).
To that end if you want to note who they are, not be friends with them, not employ them, not use their services, deny them non essential services you might run... OK then, more power to you in fact.
To deny them the right to speak though... the world has to be big enough and ugly enough to take care of itself.
You appear to be somewhat worried about it being contagious, and maybe it is, however underground it can still thrive. I will also bring up the Superman radio drama -- it had all sorts of real code words and whatever else fed to it by an infiltrator, broadcast out... membership and support fell dramatically once people were aware of the batshit insane ideas they were spouting. Prior to that they were also routinely investigated by law enforcement and not exactly allowed free reign.
Spin it another way. Religions of various stripes, caused no small amount of strife both individual and group wise throughout history, their codified rules in books they purport to follow contain some egregious shit. Big protest saying we need more religion. What do you do? It could be functionally the same thing. Worse is you know it will be one of those slippery slope things; what government in history has really ever surrendered powers (particularly ones that juicy) or not feature crept them? Go another. All those people protesting the police. Someone (or better yet a nice false flag) sees a policeman get popped. If you have that you now have those essential to law and order (not even just citizens) being killed and your protest criminalised, or at least... seriously legally dissuaded, as a result.
The though of surrendering some free speech to combat them... just no. I don't know if in this instance it means they have won but someone has and it ain't me.