Need Advice

We have three cats, who have recently transferred to being outdoors almost full time. We have a lovely heated pet house for them with hay and blankets, food and water. However, in the last couple weeks we've noticed a feral cat spending a lot of time on our deck and just generally on our property. Which wouldn't usually be an issue, but this cat is very skittish and mean. 2/3 of our cats don't mind it hanging around, but one cat is being forced out of the shelter because this stray feral cat is sleeping in their house. We were wondering why our cats had been crying for food way more often, turns out this feral cat is eating away at their food as well. When it's not in the cat house it's under our shed. We kept our one cat indoors last night because she doesn't want to go into the shelter with the feral cat, and it would be cruel to leave her out in the cold. No matter how many attempts we've made to get the feral cat out of the house, it just keeps coming back within the hour of us shooing it away. We can't afford to take this cat to a vet and get it sterilized and can't get close enough to catch it, but we need this cat off our property asap. What should we do??