Delete the junk users?

I have over 4,000 user registrations on my punbb forum that are pure junk. They have no interest in true participation, they just create an account and post links to their website to help with their SEO.

I don't think these are bots because my punbb is supposedly configured to auto-delete any accounts that don't confirm emails. I'm not sure if these deletes are automatically happening. How can I tell? And how can I tell if these users actually confirmed their emails?

Most important: Is there an sql query (or a way through the Admin UI) that will safely delete all users that have been a member for over 30 days and never made a post?

How can I solve this problem of people creating accounts just for SEO? Is there any automated way through punbb to put a stop to this? Note: I know how to delete users one at a time but that just takes way too much time. I'm getting 50 junk registrations per day, at least.

Thanks.