I used to own a forum (sorry, can't share the link here) that ran on phpBB and this was a huge problem. By the end of the month, I'd have hundreds of bot-created accounts which all had backlinks on their profile page. Some tried to spam the boards. Others didn't post anything. Eventually I had to change the forum settings so that all new posts would need to be approved by me and any accounts that had no posts in a week would get pruned. My forum had a CAPTCHA although it was the weak one that's supplied with phpBB by default.
My WordPress blogs had a similar problem too except it involved the comments instead. The problem died down once I implemented Akismet.
Proposal: disallow profile URLs for newbie members?
And delete 0 message account every X months
And prohibit send private messages. One of them sent me spam about
http://dogecoindouble.com/ 
I get spam PMs sometimes from newbie accounts too. I just ignore them and they usually disappear after a while.
Also that link is down at the moment. Looks like the site owner forgot to update their contact details.
I believe that nuking them won't do. It might remove the spam they create but spamming the forum is obviously not their goal. Deleting the account would also be problematic if it's done manually because you would be fighting bare handed against a machine. As I mentioned in a
post I made about this, those accounts are probably made so websites can have backlinks. The forum needs to be protected from such automated account creation techniques, and I know it's hard to prevent this completely BUT it could at least become less meaningful with some small changes. Let's say that account's below Jr. Member position won't be able to use the forum's features to add links to their signature and website space. Take a look at the
newest accounts. Most of them have 0 or 1 posts, but almost all of them have set a website address. Turns out the forum is vulnerable and someone is abusing this vulnerability to practice his blackhat SEO techniques.
After scanning through that list, it looks like bot-created accounts outnumber genuine accounts 9-to-1. There are some genuine accounts there though. For example, these accounts look genuine:
https://bitcointalksearch.org/user/tortuyosi-418467https://bitcointalksearch.org/user/bman84-418475https://bitcointalksearch.org/user/sherskiotis-418479 
