Big streaming services have great difficulty doing it and that's just addressing legit VPN providers. This method is 100% ineffective for forum spammers. How do you stop, for example, Hola exitpoints as just one easily available example? You can't.

I'm a noob here but I wouldn't have minded needing approval on my first x posts (even 1 or 2 would probably do it) especially if creating a thread. It's common on a lot of forums now because of this very thing. That solution is viable if mods are willing and able. But it is extra work. It's hard to get forum security without inconveniencing someone though.