I disagree with much of, if not all of what he says, but I do believe there should be competing opinions and viewpoints in every sub, especially one such as the Dev and Tech sub.
If his posts are off topic, or otherwise break the rules, they should be removed, and if necessary, his posts could be closely monitored/moderated. If his post follows the rules, and is on topic, but you disagree with what he is saying, you should challenge him, present your side, and facts to support your side, and others can judge for themselves. When there are competing viewpoints, you can say with more authority that you are correct. When you start preventing certain people from posting in certain subs that have opposing viewpoints, all information presented in the sub has less credibility.
But he doesn't just present a different view point. He repeatedly flames other users and creates off topic posts. The fact that his posts often result in the locking of threads is pretty telling. Many people have challenged him and presented their side. His responses are usually some twist of words, snide remarks, or insults and they further drag the thread off topic. There are numerous personal and direct insults in his posts.
Many of his posts begin slightly on topic, e.g. in a thread about lightning he might mention something about lightning, but then start talking about segwit (which is somewhat related) and later block sizes. People will then respond correcting the incorrect things he has said about lightning, segwit, and block sizes. Then responses begin focusing on the incorrect things he has said and repeated back and forth. So the topic goes from discussing lightning to people arguing back and forth. Such a thread has now be derailed and taken off topic. Eventually insults are thrown and people start flaming, at which point it is necessary to lock the thread.
Now consider that this has happened to multiple threads, with largely the same catalyst (same set of posters, which includes franky1, making posts that result in thread derailment and locking). Given that this repeatedly happen, I, and apparently so does gmaxwell, finds this to be in violation of the No Trolling rule.
It has nothing to do with preventing certain viewpoints from being posted, that's perfectly fine. It's that the same user constantly does it in threads where those opposing viewpoints are really relevant to the topic.
I didn't realize members could be banned from specific sections. That's good to know.
Can someone technically be subforum banned on SMF? Would like to have clarification from theymos regarding this.
I don't think SMF really allows that. It's just that gmaxwell has decided to delete any post that franky1 makes in Dev & Tech, and has announced that decision publicly. It's close enough to a ban so that's what he called it.