After playing around with Apache mod_rewrite for a while I have found a rule that seems to fix my Google Adsense issue:
RewriteRule ^\/pagead\/badge\/checkout\_999999\.gif$ http://pagead2.googlesyndication.com/pagead/badge/checkout_999999.gif [R=301,L]
The problem is I'm not sure how to stop a similar thing happening in the future if Google decides to hotlink to a different file?
Sounds like a bug with google adsense delivery. File a bug with them is your best bet for a long-term fix.
As Google doesn't care about the problem (previous posts were sent in June and we are now in October... and the bug was reported directely to Google), I decided to put an image on my server that would suggest the user to click on the ad!
Clicks then increased!!
As this is my server, I can do what I want and that's not my problem if Google asks for files on my server. They cost me money by using my bandwidth and connecting to my server, I am now paid for that!!
You should do the same...