Higgs Boson wrote in
:

Big SOB has been terrorizing household intermittently for ages.
Comes in through cat door; scarfs up cat food; messes his water
dish. We blocked up cat door; cat has to use window giving on
dark yard; have it open minimum, but anxious-making.

Google "electronic pet door". Short version: cat wears a collar with an RFID tag, door won't
unlock unless that tag is in proximity.

Or if you prefer a more DIY solution... some years ago, I remember somebody posting here
a complete description of how he'd solved the same problem by rigging up a video
camera and computer software to recognize his cat, and not open the door for anything
else. You can probably find that if you sink enough time into Googling for it.

As other posters have said, though, you should consider the various dangers involved in
letting your cat outside at all:

- Domestic cats cause a fearful death toll on songbirds.
- Cats running loose may be hit by cars.
- Cats are also potential prey for coyotes.
- When your cat goes outside, it probably doesn't stay in your yard -- if it's like some of the
cats in my neighborhood, it goes next door and s**ts in your neighbor's flowerbed. This
brings up another danger to your cat: that it may be shot by a neighbor with a pellet gun
who's sick and tired of finding cat s**t in his flowerbed and vegetable garden.

Bottom line: litter-train your damn cat, and this won't be a problem.