I had the same problem and used an induction coil on the cat food bowl. 
Doesn't hurt the cat but they never go back to that bowl again! I wouldn't 
cut the cat's whiskers (or claws) off. That is really harming them.

On Thursday, December 27, 2012 11:07:29 PM UTC-8, Raymond Weisling wrote:
>
> About 1977 I had two cats and a 24/7 cat flap, but a stray was coming in 
> during the night and getting food left  for the residents. I breadboarded a 
> cat discriminator. It used two telephone relay coils that could detect a 
> small magnet passing between them, added to the cat collars and a light 
> bulb plus detector (photoresistor). If the magnetic signal was triggered 
> and a cat entered, it was a resident, if the non-resident entered, not 
> wearing the magnet, it sounded an alarm. I added  larger flap made from 
> cardboard and a solenoid that allowed the large flap to fall and close off 
> the smaller flap so no exit was possible. The no-exit flap solenoid was 
> actually manually energized by touching two wires together on the end of a 
> cable that ran to my bedroom. Everything was rather crude. I expected that 
> I needed it once. 
>
> After I installed it I tested it with my cats with and without collars and 
> it seemed to work well.
>
> That same night at around 02:00 the alarm sounded, I touched the wires 
> together, the larger flap fell and I went out. The non-resident, hearing me 
> stirring, made a mad dash for the door and hit the large flap covering the 
> bidirectional flap. I tried to catch this panicking cat, and in the process 
> the breadboard and the lamp, photoresistor and coils all came undone from 
> their temporary mounts. It was a jumble.
>
> The non-resident had to be chased around the house, leaping up at closed 
> windows, and eventually I caught him, and trimmed off his whiskers with a 
> scissors. This is a very powerful yet harmless reminder since they depend 
> on them for feeling for passages that their body can get through. (A fellow 
> cat lover told me that once they trimmed off their cat's whiskers and the 
> can would be ware of going from room to room in the house especially if the 
> door was partly closed leaving a narrow gap.) They will be disoriented for 
> some months until new whiskers grow back. A good reminder. 
>
> I finally opened the door and released the non-resident, who seemed to 
> traverse the back yard that was a least 15 meters (or 40-some feet) long in 
> three or four leaps. He never again appeared. The damaged cat discriminator 
> was summarily taken apart. I remember using LM324 and LM 339 in the circuit.
>
> One of the cats was a great hunter, and I lived north of the San Fernando 
> Valley in foothill areas (Newhall, CA) where some ground squirrels lived. 
> My hunter cat, a gentle calico, would bring home slain squirrels and leave 
> various parts somewhere in the house as a token of her skill, for me to 
> find and clean up when I got home. This happened on a nearly daily basis 
> one spring. Eventually it stopped and I found that the nearby colony has 
> been totally exterminated by my calico. For a while I had wondered what it 
> would take to build a prey discriminator that could block her entry only 
> when she carried a victim, but even now I suspect that that is a much 
> greater challenge.

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