Step 1. Institute a captcha for all new members to prevent robots from making spam (most likely the new members that have only 4 posts, all four of them advertising some other site or produce are in this category).
Step 2. Track our IP's - when it changes, challenge us with a Captcha. This means most of us will have to solve a captcha once a week or so (some as often as 1x in 24 hours). If you wanted to be really sophisticated, track our IPs by region - if I always log in from NC and Florida (and I do, as I live between both states), and suddenly, my account is logged in from korea, you know that something is up and it's captcha time. This would help some of the existing members who seem to have had their accounts compromised (only a few...)
Assuming you wrote your own forum software, the originators of Captcha (Completely Automated Public Turing Test To Tell Computers and Humans Apart) can - along with their free software - be found at captcha.net .
Thank you for your time, and sorry for going off topic.
Ken