I might be misunderstanding what booing means then. My understanding is covering another person's voice with shouts in order to sabotage his speech. It might indeed be part of what some society might define free speech, but I'd consider it more of a coward form of violence.
If with "booing" you mean "disrespect whatever good idea a person has because it also has very bad ideas", then I wonder who we will end up respecting. Even I have ideas I end up discovering bad. Should I boo myself and ignore everything else I say?
If I am missing another definition of booing then I am sorry.
That is exactly what booing is, but citizens are allowed to boo. I can boo you, you can boo me. If you are booing me then I can walk away, and likewise you can walk away from me. If I'm booing you during a public performance that is indeed rude but then I need to be thrown out by security, which is perfectly allowed and expected.
Citizens, i.e each other, are not the problem when it comes to free speech, ever. The only entity which needs to be defended against is the entity that has a monopoly on violence, which is of course the government.