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.
No. Anything that can be automated will always make it marginally more difficult for the people you want to keep out than the Newbies that are going to have to deal with the crap.
Think of it almost like purchasing power inequalities: you can sell something for $1 and raise it to $1.50, but it affects people of different statuses differently.