They are speaking freely; it's just that what they are doing is exactly what you said - "shut down other people". It's like a sadistic little kid who likes to pull wings off flies, shoot his BB gun at dogs and set fire to cats - they gain pleasure from the pain of others.
In some ways it's like "You can't con an honest man". In this case you can't troll a person who doesn't care. Eventually they just move on to someone they can troll into an emotional reaction. That's where they gain their pleasure - by producing an emotional reaction in others.
It doesn't even have to be the person being trolled. It can be those supporting the person being trolled. The pleasure is the same for the troll.
All that said, it's still free speech. Others have the freedom to not read or completely disregard the troll(s). I'm against having an all-powerful force dictating what is and what is not "trolling" since such human decisions are inherently biased and often lead to even more bias.
Please notice the power of Individual choice. It is my strongest opinion that empowering others to limit choices to "We, the People" is a path that always ends badly. Better to suffer assholes and be free than to be free of assholes and oppressed.