Hi
This should do it, you place the robots.txt in the root directory of your site.
User-agent: * Disallow: /product-category/
You can check out some more examples here: http://www.seomoz.org/learn-seo/robotstxt
As for the multiple urls linking to the same pages, you will just need to check all possible variants and make sure you have them covered in the robots.txt file.
Google webmaster tools has a page where you can use to check if the robots.txt file is doing what you expect it to do (under Health -> Blocked Urls).
It might be easier to block the pages with a meta tag as described in the link above if you are running a plugin allowing this, that should take care of all the different url structures also.
Hope that helps!