I didn't mean blocking them from using the site or expressing their mind, my bad, I didn't make myself clear enough. What I was proposing is more like how we block users on twitter, where we don't see any tweets of theirs from next time, by default. Of course, we can still unblock them for a moment if we wish, but in general, this helps with peace. That blocked user's tweets are ofc still visible to everyone else who hasn't blocked the user.
A random anonymous forum often brings out the worst in people, and people abuse their freedoms with no accountability. I don't propose auto-banning ofc. But the point OP raised about how it becomes cumbersome for a few mods, is quite genuinely valid. This would reduce some toxic spam threads, since people wouldn't fall for these users' bait threads, since they'll just get blocked after the first time. This would incentivise the spammers to stop spamming as well, since they would get lesser responses. A reputation system is an easier way to solve the problem as well, but that's a different discussion :D