If someone joins and never posts, 90 days is plenty of time to keep 'em and the delete 'em.
I know that there is a record of last post, is there also a record of the last visit? If so, that would let you know if they have been lurking and reading.
For those that have posted and then stop, I suggest an email to them inviting them back. That could be a 90 day time period for the email to go out, then another 30 to return. No return, well, I guess they can rejoin if they want.
Sidenote: I read an article about forums, and the author suggested that people join and read for a couple of weeks before posting. That can be good and bad......
PS I'm glad you don't allow anonymous posting; IMO, too risky.