Well that was kind of what I was asking. What do other people do? This must be a common problem. :S
One of the sites I run and administer has over 3000 registered users, but only about 25 have actually logged in recently, and the assumption is the rest have lost interest or forgotten they had ever registered. There may be GDPR implications of keeping dormant forum accounts enabled.
So, the obvious solution is to send an email to any registered user who hasn't logged-in for a specifiable time (eg 1 year) and capture any bounces and make the users inactive if the emails bounce. Then after a further configurable time, eg a month, make inactive anyone who still hasn't logged back in after their previous reminder or replied to an email
Since Community Builder doesn't appear to have a solution natively, I'll just use the Joomla add-on RegReminder.
Thanks
Brian