Sorry, I thought the answers you sought were obvious to anyone following the news.
See [1], [2], & [3] for some examples, all of which involved organized groups doing premeditated actions to stop speech or disrupt a concert, in one case with mass murder. And of course TFA, about the serious premeditated security threats to a famous performer.
It's apparent that we're talking about two different things.
In your anecdata set, a heckler is apparently an over-drunk loud-ass in a comedy club. Sure, that's a "so what?" event.
I'm talking about the much larger issue of actual premeditated and/or organized disruption, which does exist, even though you "have never heard of it".
I'll agree that there are MANY good arguments to regulate and manage use of facial recognition tech, having to do with sound security (vs. security theater), individual rights & avoiding improper discrimination, general human rights privacy protections, and even maintaining normalcy as you noted in kind of an afterthought.
However, your "spontaneity" reasoning is especially weak and unconvincing, despite your apparent feeling that it should trump all others. The ad-blocker comment was specifically a rhetorical analogy and definitely not intended as ad hominem; it is interesting to see that you took it that way (if I wanted to make an ad hominem argument, I'd say that the 'spontaneity' arg sounds like someone from a sheltered background who doesn't get out of their bubble to read news, but I don't bother with ad hominem arguments).
For the "criteria for heckling?" question, I'd refer back to the previous answer, when a known person or group would disrupt an event to the extent that it destroys the spontaneity of the rest of the audience.
Slippery slope? Likely, but there are obviously degrees, and easily recognizable ends where it's appropriate to exclude or not. I think we'd agree that shooting the performer would be very exciting, but quite out of bounds. Wearing a funny hat, as long as it isn't too tall so as to obstruct the view of those behind is in bounds. Stopping the performance, out... Stalking? out...
I'm willing to recognize that there are both valid use cases, and arguments against, and that it needs to be tuned.
[1] https://www.usatoday.com/story/news/world/2018/05/22/manches....
[2] https://www.washingtonpost.com/news/volokh-conspiracy/wp/201....
[3] pretty much any event with Milo Yiannopoulos, some of which have nearly ended in riot. While I object to almost his entire attitude & view set & would likely agree with the protesters, when he's booked in and people want to come see him speak, they want to see him, not the protesters, and it seems to me that he ought to have the right to decide whether he wants them outside or in.