Cat owner wanting to deter stray cat!

MBin

I got a cat last summer. Since I got her a stray cat has started to hang around. I am sure it is stray as it's always starving and has no collar and won't come near anybody. My cat is scared of this cat and it has already hurt her, so badly that I had to take her to the vet as she got an infection from a bite. My cat is mostly an indoor cat but I go home every day at lunchtime & leave her out then back in when I get home after work. When I let her out I leave the cat flap open which gives her access to the utility so if she wants to come in if it's raining etc she has somewhere to shelter, but as far as I can tell it's the stray cat that's coming in for shelter! It was fast sleep in her bed when I got home yesterday and only woke when I opened the door to the utility, which gave us both a scare! At the weekend when I'm home & I leave the cat out I often see the stray cat sneeking in and chase her off. I never leave food out that this cat could get to, I feed my cat in the kitchen & put the bowl away when she'd finished eating. But on occasion if I was home and had let my cat out and had the door to the kitchen open it has come in through the cat flap and utility and jumped up on the counter and got something or bullied my cat from her food bowl when I wasn't looking. Every time I see the stray I shout at it and chase it off. I can't throw water etc over it as the minute it sees you it runs and generally it's gone back out the cat flap and gone off by the time I get the door open. Obviously I can't be spraying stuff around the place etc to deter it as I don't want to upset my own cat. I've called limerick pound and a few other numbers they gave me but nobody seems to deal with catching stray cats, only dogs. I know you can get cat flaps that only allow access to the cat that has the magnet on the collar, this is the cat flap I have but my cat is still too young and isn't strong enough to open the magnetised door yet so when I leave her out during the day I have to fit the door open fully. Does anyone have any suggestions of how I can stop this stray cat coming around?