just like one of those catflaps that won't let a cat in without a magnetic collar.
except this one lets them all in, but if they don't have the collar a guillotine blade cuts them in half. The prototype's working pretty well but I haven't solved the mess issue yet
Someone has pipped you to the post, though he didn't involve the use of a guillotine, but an orbiting 1000 Megaton peace keeping bomb.
"Photon Push-Pull Radiation Detector For Use In Chromatically Selective Cat Flap Control And 1000 Megaton Earth-Orbital Peace-Keeping Bomb". - http://en.wikipedia.org/wiki/Arthur_Paul_Pedrick
"Many of Pedrick's inventions related to his cat, and trying to improve the life of his sadly aging moggie. His crowning achievement in this respect was patent GB1426698 titled "Photon Push-Pull Radiation Detector For Use In Chromatically Selective Cat Flap Control And 1000 Megaton Earth-Orbital Peace-Keeping Bomb".
The idea was to detect the difference in fur colour between a ginger cat and a black cat. He came up with this idea because a black cat named "Blackie" from next door kept trying to steal his own cat's food. However, with Pedrick's new catflap design, if Blackie attempted to use the cat-flap he would not be allowed in. Ginger was impressed with the idea, and further suggested that the concept could be applied as a nuclear deterrent. Ginger's concern over the Cold War and the nuclear arsenals threatening the world was a regular motivator for Pedrick's inventions."
[url]http://worldwide.espacenet.com/publicat ... &KC=A&ND=1