There is a limitation of free speech - fighting words. Over time, this has become more and more narrow, including Snyder v. Phelps where Westboro Baptist Church was ruled to have protected speech despite their words being pointedly designed to provoke a reaction from the people they are yelling at.
In light of some of the reports from Trump rallies, at least some of the protestors (or disruptors, to head of the semantic discussion) are clearly near this line if not over it.
I'm not really sure what I think about that. I don't think there is anything anyone could shout verbally that would induce me to take a swing at them. But I do have some sympathy for the concept, especially when those words trigger deep-seated emotional reactions.
"What did you say about my Momma?"
I've never appreciated or supported this kind of angry, provocative yelling, especially getting into the personal realm. Like the folks who would (and still do) scream at people entering Planned Parenthood clinics.
I don't think I'm willing to make it illegal to do that, but man I think there are much more effective ways to make your point and try to bring people to your side.