There's no constitutional rights when it comes to privately owned boards. I've actually seen people writing that "they have a right to an opinion". Not if the admin don't like what you say you don't.
Whenever someone has said to me that they can say what they like as it's their opinion, I've always replied that they can have an opinion, but they don't have to express it.
I once worked with an old crone who considered it a victory if she could upset someone enough and make them cry. She didn't like it when I told her that I was fed up with her insulting people and could no longer work with her and wanted to transfer to another department, and she then reported me for insubordination!
Going to my school was an education in itself. Which is not to be confused with actually getting an education (Schultz)