Good luck and don't lose hope. [edited by: Steph_R at 12:36 am (utc) on Aug. 3, 2006]
Before you jump to conclusions, I would check the obvious reasons: does your site have duplicate content (or even duplicate meta tags), multiple domains on the same site (parking?), server issues, are you linking to other sites that you control or own, did you neglect to setup "href no follow" on the links that you allow your visitors to post on your forum, etc... I am not 100% convinced that the "spammer" you described could do this much damage. If you find the cause of the problem, you can fix it then do a re-inclusion request.
Good luck and don't lose hope.
[edited by: Steph_R at 12:36 am (utc) on Aug. 3, 2006]