Depending on where something like this (cancelling a person, basically telling them they’re a social outcast as indicated in the linked comic, etc) happens, it wouldn’t necessarily violate constitutional rights, but it may qualify as emotional abuse and some places consider emotional abuse a crime equal to crimes like domestic abuse. It could also be fought in civil court where such a suit would not be considered criminal, which would likely be charged as “Intentional Infliction of Emotional Distress.” It could be argued that the person is being treated too harshly or something, but that would be on a case by case basis.

Just because a constitutional right is not being violated doesn’t mean something is right or okay to do, or that you are somehow safe from legal repercussions if the person decides to sue over it. Of course, the same legal action is also available to a person that feels offended/hurt/distressed/etc by something someone else says. You can sue for anything, afterall.

Businesses, just like people, also have the right to trespass people off of their private property, and I would imagine that forums and message boards of companies would be legally considered like such private property. So if someone is violating the rules, maybe they could try to sue for emotional distress but that case would be near impossible to win. The business/forum owner has the right to tell anyone they want to leave for any reason they want. Thats how it is.