This morning, Google detected and alerted me that something redirecting users had made its way onto one of the ad networks I work with. Until I determine exactly where, and what it is, I've disabled all non-Google ads on the site.
For now, when visiting through google search and using google chrome, there is a warning about our site being deceptive - on every page of the domain, including the forum. I will not re-enable ads until I know that the culprit has been removed, and should any network prove to be a habitual offender I would cease working with them.
This is a very frustrating experience, but I'm glad that Google does this as it'd be even worse for something to go undetected and cause real people real problems.