Theoretically, the canonical is supposed to work, but I have seen situations where the incorrect version of URLs still end up in Google's index, even with a standard canonical. I do not know if those are having a negative impact, but their mere presence in the index suggests that it could. Google sometimes tends to put an URL in their index if they find it in a hyperlink somewhere. These should eventually get filtered out, but if they were paying strict attention to the canonical tag, the link would never show up in their index.
Bogart's hash mark trick should definitely work, but they will need to capture the reference code when the page is loaded. I have not seen HTML anchor bookmarks (URLs with hash marks) show up in AwStats or other standard log file analysis software.
There is a Google help page out there that I cannot currently find that lists reference parameters that Google ignores. They are mostly session IDs. That suggests that making the reference look like a session ID might work, but probably only with Google.
It also appears that Google Webmaster Tools just added a feature that allows site owners to specify parameters that should be ignored.
If the forum owners know anything about SEO, they could be capturing the URL, storing the reference code, and then rewriting the URL with a 301 redirect. If they are doing that, you would see the reference parameters get stripped out of the URL in the browser's address window. Basically, the reference code is being stored and then stripped out with the 301 redirect.
If they are not doing anything to deal with the potentially negative SEO issues, they are probably making a very large mistake.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin