Site Over-Indexed

Site is etting over-indexed by google

[seoer]

I have a php site (we'll call it xyz.com) and it's getting over-indexed by google and it's costing me too much in bandwidth.

Here's my problem:

I only have 20 pages on my site and none of them are very large ( < 100k for sure). However, I have set up advertising partners on quite a few sites and they are referring to me by using the url:

www.xyz.com/index.php?b=advertising_partner_name

Now, when google indexes me, it indexes my site for every single permutation of each of my pages and each of my advertising partners. So:

Google indexes:
page1.php?b=advertising_partner_1
page1.php?b=advertising_partner_2
page1.php?b=advertising_partner_3
page2.php?b=advertising_partner_1
page2.php?b=advertising_partner_2
page2.php?b=advertising_partner_3
page3.php?b=advertising_partner_1
page3.php?b=advertising_partner_2
page3.php?b=advertising_partner_3

... and you get the idea.

A google search for "site:www.xyz.com -asdf" reveals 3000 indexed pages for my 20 page site! I'm surprised Google doesn't correct for this!

But anyway, does anyone know how to solve this problem? Perhaps through a robots.txt file? But I don't know how I would go about excluding all pages that follow after a "?". And I definitely don't want to just exclude my entire site.

In case it helps to know, I absolutely do not have any internal links with the "?b=" in the URL. Those are only used by my external advertising partners.

Any help would be appreciated. Thanks.