ReCaptcha helps a bit. I'm using PHP Fusion 7.x and accounts are still being created on my site. I have turned on requiring admin approval before account activation and it stops the flow. Oddly enough, the spam accounts that one of the other admins activated did not post on our forums or any content on our site.
I am in the process of making a simple horizontal slider bar with values ranging from 1-100. Every week, the server will randomly determine the required value and will force users to slide the bar till it reaches that value before they can create an account. I don't know if it will help but I thought it was worth a shot.