Are you the sponsor or a webmaster trying to block it?
You have to set the user agent to Google, to use a wild card in the disallow.
Code:
User-agent: Googlebot
Disallow: /*?nats=
Disallow: /index.php?nats=*
If you are the sponsor program. Put the robots.txt file on your money program domain that has your nats/www files. Then you can kill the redirect scripts value (which is the bad thing) and google can choose to use or ignore the duplicate link with the ?nats var or count it's value.
If you block at the ?nats= level, the links won't have any value, not even a link count value, I don't think. And any links - value or not, is a good thing.
Code:
User-agent: Googlebot
Disallow: /track/
Disallow: /strack/
Or if you want to hard link your galleries, to keep the best value. You can use a php script to look and see if ?nats is passed in or not. If so, use that nats code. If not, use a default code from the php to build the links to the paysites.