is it possible to fake a referrer for adsense?
That's a pretty complex question, especially when it comes to discussing blanking vs faking. Ads3ns3 is the one advertiser where you will most definitely attract a ban if you send only blanked referers - no questions, no excuses, all blanked results in a ban.
As for faking, there are 2 referers you need to be concerned about with Adsense:
a) The site the ad is being clicked on (let's say cargadgets.com), and
b) the site/s that send traffic to a, above (i.e. send traffic to cargadgets.com, which has the ads3ns3 ads).
It is virtually impossible to fake "a", as the ads3ns3 code is hosted by G, so they "know" where it is being displayed. It "can" be spoofed, in a way, using invisible windows and the like, but you need control over the site in "a", as it will still show up.
"b" can be spoofed. If you have ads on cargadgets.com, then adse3ns3 tracks where cargadget's traffic is coming from. If you are sending your traffic from a "junk" traffic site, you "can" send it via a fake ref script to make it appear as though it's coming from a whitehat site.
Nevertheless, ads3ns3 is one of the hardest advertisers to manage to get shady stuff past them, so you would need to seek out threads which specifically deal with the subject, in order to learn how to adequately cover all the angles.