First, freedom of speech does not mean freedom from consequences.
Second, all sides (liberals, conservatives, moderates) have trolls and jerks among them.
Third, if it's a forum run by a non-government source, the First Amendment doesn't apply. They don't have to let someone (or anyone for that matter) use their forum. Especially if they break the rules of the forum.
Fourth, there is a difference between open debate and toxic posting. I fully believe in open debate. You might not change my mind on a topic, but I welcome an honest and open debate. But some people don't want to debate. They want to shout down, or harass, or whatever. And unless you have a crack moderator staff, you can't always keep on top of this sort of thing, especially if it's a large/popular forum.
I speak from some experience on both sides of this. Some years ago, I was a moderator on an MMO forum for Dark Age of Camelot. It wasn't a forum run by the company that made DAOC, it was a fan forum. I ended up as a moderator (much to my surprise) for regularly giving helpful information and trying to be a good guy. While I was a moderator, we had to put people in various lengths of "time out" for being jerks, or abusing the forum rules, or spamming, or whatever.
Shockingly, the moderators weren't always the most popular people with the friends of the trolls.
On the other side of the fence, there was (possibly still is) a forum for a group called Christian Exodus. Among their stated goals is (as a last resort), the idea of moving enough followers to a specific U.S. state (South Carolina), to try and influence enough elections and legislators to make it a more Christian state. Actually, their absolute last resort was to have SC secede from the U.S.
I was banned from their forums because I would do things like remind them that the last time that SC seceded, it didn't go all that well, or remind them that there's more than one version of the Bible (they were very focused on returning to doing _exactly_ what the Bible said), and so on. I was enjoying myself, but I can't really say it came as a shock when I was banned from posting on their forum.