I would put the Ajax function into robots.txt:
That will prevent Google from crawling it. Google doesn't typically index URLs it can't crawl. It will only index them if they are linked prominently, especially numerous external links. Even if Google does index the URL, it won't index the content of the URL. Google will show "this page was blocked by robots.txt" in the search results.
On an Apache server you can add that header with .htaccess code like:
Header append X-Robots-Tag "noindex"
In aspx code you can set it like this:
<% Response.AddHeader "X-Robots-Tag", "noindex" %>