What is the correct way to hide an external link from Google's bots? i.e. I have lots of links to a particular e-commerce site, but I'd rather Google didn't kn

What is the correct way to hide an external link from Google's bots? i.e. I have lots of links to a particular e-commerce site, but I'd rather Google didn't know about it so my site doesn't get a ranking penalty. Currently I have links to a certain directory on my site, and that entire directory is "Disallow" in the robots.txt. When the index.php in that directory loads it 302 re-directs to the e-commerce site in question.