Do you mean the legal definition of harassment, or just the colloquial one? If the legal one, then use the legal system to stop it instead of vigilantism. If just the colloquial one, then it is a free speech issue.
there's specific prohibitions against discrimination on the basis of certain categories
Where/how is vigilantism occurring?
There doesn’t need to be a legal case to refuse service to someone using your services for harassment.
If a group of people make a hobby out of walking into gay bars and verbally abusing customers, it’s not vigilantism to refuse service to those groups / kick them out.
One can be banned from an establishment for far less.
Moon has its own server, its own network (AS), a colocation that is willing to host his server as long as he does not violate US law or ignores incoming complaints.
But now it's the peering companies just refusing to route to them, based on some complaints that might not even be fact checked. For many that could be considered essential infrastructure, since you cannot simply start your own peering company.
For reference, his .is domain was taken down since people complained about CSAM hosted on his site. But in fact the links provided were not CSAM. But who is willing to check with that and potentially having to deal with law enforcement as a consequence for this? Likely no one, thus they drop him to stay on the safe and easy side then to protect a costumer that's could be trouble, even if some accusations are wrong.
It's a bit similar as with roads or with the post. Many people dislike Joe Rogan I guess. But imagine now disallowing him to drive on certain roads, UPS/DHL refusing to send packages to his address, AT&T disconnecting his lines because they disagree with him.
Surely at some point you cannot just build everything on your own.
And that's all for services that are (still) considered to be legal, but for which companies just are not willing to take up the heat.
I agree that the participants there are engaging in something closer to vigilantism, although I don’t think they actually believe they have a righteous cause, they’re just unabashedly intolerant and enjoy harassing people which is something else.
I would of course also love to see this addressed in legislature, but that seems unlikely given the current political climate.