As for spam, I guess I was thinking more along the lines of making sure that no one had snuck content / pages on to your site that had "pharmacological" material on it.
As for canonical, what I meant was whether your pages can only be indexed one way by google. I know some sites that use tags for navigation or have pages that show up in multiple categories, so a single page could show up as:
so I would do a site:mydomain.com in google and see whether the individual pages are indexed multiple times with different URLs.
(I know, this is kind of basic, but maybe it will help?)
Were there specific dates when traffic declined sharply? Or was it just a gradual decline over time?
If you saw sharp declines in traffic or in your number of impressions, then note down the days of the sharp declines.
I am sorry to hear that the UGC was too hard to maintain. But you can probably guess by the number of moderators that WebmasterWorld uses here that it takes a lot of time to really keep a UGC site up to snuff.