I disagree. I'm a moderator at /r/AskHistorians, and we have a strict "no conspiracy theories, no misinformation, no Holocaust denial" policy, which works extremely well for us. Here is a Slate piece one of our other mods wrote that explains it all better than I could.
The problem is first that these groups were allowed to get a foothold on mainstream social media sites. You're right that when they get banned, they double down on the conspiratorial thinking, but before they get banned they get hundreds of new converts to be then convinced there's something real by the banning. If social media sites took them seriously at the outset, many fewer people would go through this process, and would now be happily engaging in non-conspiratorial recipe-sharing or whatever.
The second problem is that they don't use human moderators, or they hedge in their (already unhappy and unmotivated because they have to look at shit all day and don't get paid nearly enough for it) human moderators with stupidly strict rules to make sure that they're only banning the minimum of content that could be considered part of the conspiracy, e.g. removing posts that say vaccines cause autism but allowing people to talk about ivermectin as much as they want.
Fact checking isn't a pregnancy test. It's not about getting a Yes or No. The most dangerous misinfo is at least partly true, and requires an explanation of how and why it's wrong.
This is not wrong, but. It is very much the case that trying to rationally correct these people simply leads to a gish gallop of more wrong info than anyone can reasonably correct, and they are never, ever convinced by it. In a more open forum, the explanation can at least be seen by people who haven't been radicalized and used to protect them against being radicalized later, but in a Facebook antivaxx group, the likelihood of that happening is so low that it's non-existent. I don't know what sites are or aren't doing, but linking to a pre-written explanation of the problem that doesn't allow comments is a good way to split the difference - explain the issue with the misinfo but don't make it a forum for debate.