Free speech is a "privledge controlled by authorities" as many anarchists have pointed out, but beyond that, especially when planning disruptions of fascist speaking events as you are directly referring to, there are a couple of conundrums:
-how does disrupting a particular fool's or mob's deranged expressions benefit a more lawless and anarchistic future?
-how much do we risk galvanizing the fascists? This is actually impossible to predict, which is why I don't have any personal interest in showing up on the streets with right wingers, or calling them out in a very confrontational way. IMHO, the activities of self-proclaimed "anti-fa" in recent years has only strengthened right-wing support, and at best has made certain people "afraid to be racist", which at any moment could turn into reactionary or authoritarian activity.
-Anti-fa is theoretically a very dangerous type of activity, for example, if you disrupt one of those hardcore neo-nazis from speaking, you might end up getting your ass kicked, which is exactly the type of expression they [the nazis] are looking for.
But beyond that, these kinds of questions about disrupting gatherings are impossible to answer. It almost relies on a crystal clear reading of the future: Clearly it's a good idea to disrupt certain gatherings and speakers, clearly it's NOT a good idea to disrupt some events.
This question reminds me of an event that Kristian Williams tried to host at a university, it's disruption was on youtube for a while. The people in the crowd were responding to this well written and thoughtful essay:
the event was successfully disrupted, but clearly the people who did this were just being assholes, this was machiavellian censorship in it's purest form, an attempt to once again cast women as the perpetual victims of trauma, and cast kristian williams as an authoritarian and a misogynist. He was there to discuss his anti-cop book but the conversation got sidetracked by people with a different agenda.
This is why i tend to find public speaking overall a very questionable route for anarchists...there are more or less two sides set by design: a speaker, and a group, yet i am not against it, i just find it questionable and somewhat inherently opposed to a more dynamic and fluid anarchy.