No the content of Donna's post is her domain, this website is Jim's domain. Donna can flaunt those rules and possibly or not break Jim's rules and Jim is free to remove or not remove Donna as he sees fit. In either case the Constitution has nothing to do with it as far as the rules go. Jim can refuse posting privileges as he sees fit. He cannot violate her free speech as he is not government and not bound by the Constitution, by the same token Donna can make her own website and refuse Jim posting privileges and as long as neither has broken any laws they are within their rights.
The point being that I can refuse entrance to my house to anyone I see fit and I can limit their speech as I see fit and they can do the same to me. I am not bound by the Constitution only the government is. I am bound by the laws of the land as we all are. That is why it is so important that they be Constitutional.
But you can't come into Jim's house and limit the speech of his guests...that's what Donna wished to do.
The content of Donna's post is NOT her domain, it's Jim's. Otherwise, Donna would have the ability to censor, or even pull individual posts, or even the entire thread. She doesn't.
Under the informal "rules" of the forum, articles are posted, then people are given the opportunity to freely comment, this ability is given by the site's owner, and can't be negated by Donna because this is "her thread".