How do i stop my dogs from eating cat food?

I have two dogs - a brittany and a yorkie, and a cat - Keats. The dogs eat their own food, and then, the yorkie in particular, go to eat Keats food. She is therefore quite overweight, and we know that to fix it we have to stop her from eating the cat food, however I do not know how. Putting the food up somewhere high is out of the question because Keats is 16, and he has arthritis. He cant jump anywhere. What do you think I should do to stop the dogs from eating his food?