I have just been accused of exactly the same thing!! Apparantly my 6mnth old female kitten has been jumping into next door...through her kitchen window, and spraying up her walls and eating her food. She is booked into be neutered next week as it goes, as shes only just turned 6months, so hoping if it is my cat this will resolve it anyway. But i find it HIGHLY unlikely that its my cat. She cant jump that high yet for starters, and always eats her own food... i dont know. But my neighbour needs to get a cat flap and keep her window shut!!
Might try the above suggestion of keeping my cat in for a couple of days...or tell my neighbour to trap the offending cat next time and see if it is mine!