I don’t even know if there is an answer, my female cat is 13 yrs old and really shy and nervy. I’ve susoected for ages that another cat has been coming in the cat flap and eating her food. I can’t put one of those collars on her that trigger the cat flap just for her as she somehow l9es all her collars so I gave up, I’ve literally just now, whilst sat in lounge, heard a huge hiss in kitchen. Race there to see a blooming huge long haired toirtiishell cat scaring my own cat from her own food! Chased him/her outside but what can I do now?
If the intruder cat is huge, is it so much bigger than your cat that you could try and make the cat flap smaller so your cat can get through but that one can't? Cats seems to be able to squeeze through all sorts but if it's that big a cat I figured it might be worth suggesting.
Another thought is can you get a cat flap which operates on a microchip in the cat, rather than on a collar? I'm sure you can get ones which can be paired up to your cat's chip.
Woofy's suggestion s the ONLY one that will work. The other cat will not be put off by there being no food available if it is like the one I had terrorising my two cats years ago. It came in to spray and announce its dominance. Smow's not at home all day so Woof's suggestion is the answer, that's what those particular cat flaps are made for i.e. to keep other cats out.