I've registered to forums that made me wait a few days before I could post. That's terribly annoying! But if you're a spammer who registers hundreds of accounts, the 14 days don't matter at all. Take a look at
loyce.club/patrol/: almost all of the first posts made by 100 new users are spam. And that's
despite the current restrictions on new users.
But what if we reduced it to 1-3 days and on top of that, they have to do a bunch of things before they can post, maybe like 1-3 daily log in requirement, Bitcoin address requirement (It doesn't have to be their BTC that they are using, it's like for verification only), Bitcoin message signature (maybe if it can be somehow be checked automatically). It's like email verification but it's Bitcoin address that we are verifying.
For spammers, this will be a very tedious thing to do than finding a clean IP and spamming registration on that IP.
This will give new users 2 options, do that or pay the fee.