I dont know how this forum is designed, but I guess all data is stored in a SQL database?
If so, it should be trivial to write a SQL script you can run to delete all users not logged in for the last 6 months who have posted less than eg. 2 messages. Yes, you will probably delete some 'false positives' as well, but not many active users..

I run a small-scale forum for our local sailing club, and lately commercial postings have been posted to it. I suspect that some new "spam" software for automatically posting to forums have been made. Very annoying!