You could try making an empty link with a class and the having a script run onload to create those links properly.
The below article is copied from: http://www.seomofo.com/ethics/using-javascript-to-hide-links.html
1. Remove affiliate links from your pages’ HTML code.
2. If it’s a text link embedded in page content, then just remove the <a> tags
and leave the anchor text there (as plain text).
3. If it’s an image link, remove the <a> tags AND the <img> tag.
4. Replace the <a> tags with <span> tags, and assign a class name to them
(e.g. <span class="affiliate">).
5. Write a JavaScript function that looks for <span> elements that contain
class="affiliate" and replaces them with your affiliate links.
(Yes, the ones you removed in Step 1).
6. Put the JavaScript function in an external .js file and block Google from \
accessing it (using robots.txt).
7. Attach the JavaScript function to the onload event, which means users’ web
browsers won’t call the function until after the page is finished loading.
8. Googlebot–and users without JavaScript enabled–can’t call the JavaScript
function, so your affiliate links are never inserted into the page content.
9. Only users with JavaScript-enabled browsers will see your affiliate links.
Everything/everyone else sees plain text instead of affiliate text links,
and an empty space instead of affiliate image links.