Countries such as Germany have banned certain kinds of hate speech (Holocaust denial, incitement to racial hatred, ...) for decades. There has been no slippery slope that this got more and more, it just has to be well defined and you need a serious judiciary and a healthy societal dialogue about what is and isn't acceptable. There is a far right party that everyone believes is pretty much the Nazi party, but it's not been banned as the evidence has not been strong enough. They didn't explicitly/provably violate any laws so they haven't been banned (yet).

That's a good, functional judicial process. It's a matter of professional judges and due process.