I wonder if you've thought this through properly. I suspect that, if your vision were to be enacted, there would be no more forums. No more Facebook, Twitter, Reddit, Hacker News, niche PHP forums, comment sections, etc, etc. Why? Because they'd devolve into spam and/or people arguing past each other. For example, given your current definition I believe it would be acceptable for someone to write a script to post useless replies to every single Hacker News post and comment, effectively rendering the board useless.
Right now, HN has the right to delete those. If it was a common carrier, it would presumably not. Arguably they're spam, but I cannot imagine a way you can define "spam" that is narrow enough to not be redefined by everyone as "things I disagree with", but broad enough to capture someone posting excessively to a forum. Note that this wouldn't violate the CAN-SPAM act because it's not advertising anything commercial.