The authentication scheme is there to prevent mass-manufacturing counterfeit cups. Hipsters who modify devices are below noise level on their bottom line (them bragging about sticking it to the Man is free advertisement for the brand so even if there were a loss on a machine it's hard to tell if there is a net loss or profit).
Cut power to the heat source/declaw the perforator/plug up the ink jet. I have physical access, after all.
Keurig is in the position that they can attach a number to each one of their coffee cups, and the machine will refuse to brew if the number doesn't prove the cup is authentic. If they give all the cups the same number, as they apparently have chosen here, than all anyone has to do is present that number again, and voila, the coffeemaker will execute whatever cup they feed it.
Maybe they get smart and give each and every cup a different password. Of course the machines have to recognize these passwords, so they have to start with a known list of length N, where N is the total number coffee cups they ever expect to sell for this line of machines. They put all these passwords through their favorite one-way function, stuff the hashes in a newline-delimited text file, and hope it fits in a few gigabytes. Now once the machine encounters a matching password, it brews one cup, but "crosses off" that password and won't brew for it again.
A laser would burn part of a strip for each credit used.
http://technologyandarchives.blogspot.co.uk/2011/05/how-bt-p...
(There must be better descriptions of them out there).
If Keurig wanted to they could get plenty of ideas from the consumer printer industry, where cartridges have embedded chips in them. Apparently the latest models include some form of crypto too; the older ones were just an EEPROM and were fairly easy to defeat (http://eddiem.com/photo/CIS/inkchip/chip.html) But they should also keep in mind that despite all these countermeasures, plenty of refill kits/aftermarket cartridges/chip resetters/etc. continue to be available, so they're fighting a losing war.
I figure one simple scheme would be to 'burn' the key after it is read. i.e. physically disable the DRM ink by heat/perforation/other ink, so that once used, the signature ink cannot be reused. Curious what other HN-ers would come up with. And hoping Keurig doesn't get any ideas from this. ;)
[0] http://www.theverge.com/2014/6/30/5857030/keurig-digital-rig...