[Skyrider]

If the section isn't accessible by guests, it shouldn't be searchable by google. I usually have such sections for administrators (and mods) only. Is the section public?

Also, if you wish to remove it: http://www.google.com/webmasters/

Register your site there, and add a removal request.

I don't use robots too often, once set up I'm leaving it as it is.. but you 'could' try:

Disallow: /forums/xx

EDIT:

I just tested it out using google webmaster robots.txt tester, and by for example blocking:

Disallow: /forums/32

with 32 being the section ID, I get a "blocked" message when I test the URL. Even when after the sectionID numbers show up, the section still gets blocked.

If you don't use the friendly URL and using the default of vBulletin, the disallow example would be:

Disallow: /forum/forumdisplay.php?f=32

The problem however that with this block only that specific (or more sections, depending how you set it up) will not be on the google results. But I can't say the same thing with the existing threads in those sections you wish to block, as the sectionID's are not listed in the threads URL.

But as I suggested, sign up your domain in google webmaster, and you can do tons of testing (including robots tests) and URL removal requests, etc.