What is the problem you are trying to solve?
If it is only spamming, how about a probation period before new accounts can post? No further hurdles than that. The most recent post is from 40 minutes ago, posted by a 42-minute-old account.
For a spammer, having to create an account with a password and store those credentials and come back a week later to post is far more troublesome than the current instant gratification.