A threat is mere words. Speech is speech regardless of its origin or any real implied action at a later time. I don't think it's right to say that you can restrict someone's ability to make threats, you can only restrict their ability to carry out threats. This can be done in a manner of ways, some more pragmatic than others. You can have police stationed at every single office, home and public place, for example. You could also allow people to carry arms in order to defend themselves in light of any attempts to carry out any sort of threat of violence.
If it's something like 300 people in a country of scores of millions calling for overt capital punishment and restriction of free speech, then it is generally pretty ineffective. Saying people aren't allowed to express themselves usually radicalises them and their community even more and drives them further underground.