Blocking just one parameter via robots.txt

Is this even possible?

[werty]

I am having a problem with a site serving up duplicate content. What it does is put a separate parameter on after the regular product URL, here is an example:

product:
www.*.com/index.asp?PageAction=VIEWPROD&ProdID=75

product if you click image:
www.*.com/index.asp?PageAction=VIEWPROD&ProdID=75&HS=1

Is there a way to use the robots.txt file to block this HS=1 without putting a line in for each product?

Or do I need to figure out the sites template and find out why it is adding this extra parameter on the product?

Thank you

[SEOMike]

find out why it is adding this extra parameter on the product

I'd say that's where to start. I can't think of an easy way to do what you want via a robots.txt.