If someone is harassing me and not engaging in good faith, I should be able to disengage from them and hide myself from their view.
If I was talking to someone in a park and a third person joined the conversation that's fine. If that person starts being an annoying asshole, I should be able to walk away from the harassment while still maintaining my conversation. Accepting harassment is not a requirement to talk to people, and I should not have to accept harassment from whomever wants to fuck with me for the privilege of talking to people who aren't harassing me.
I also don't consider a site where people shitpost memes to be needing the same "public forum" protections of say a town hall meeting or a politician's official communications.
"Open air free-for-alls" as I am reading you seem to prefer tend also to drive out people with marginalized identities as they leave them open to harassment people from dominate groups members do not get subjected to for just existing.
Further, there is no moral or technical reason a person should not be able to send out a message to "Everyone in the world except for Tom when he is logged in—because fuck that guy."