Neighbourhood cat fouling house

Question
I have a small cat who recently I have been leaving the conservatory window open for her to come and go as she pleases (entry into house is not accessible). However we were informed that a different cat was coming in and eating the food we had left for our cat. We stopped leaving food out but now the cat which enters urines and fouls instead. Is there anyway we can stop this happening without having to install a cat flap.

Thanks

Answer
Hello.  Unfortunately, there is no way to allow one cat in while keeping all others out, besides installing a cat flap with an electronic key.  I, myself, would opt to keep my cat indoors, and keep the window closed when I could not be home.  She is much safer indoors than out.