How do I keep my mom's cat out of my cats' food?

I moved home four months ago, after living on my own for a year, and brought my two cats (1 year old each) home with me. The problem is that my mom's eight-year-old cat doesn't want my cats in 'her' house, so she's mean to all three of us. And she's figured out that my cats are free-fed, and she's found where their food is kept. We've tried putting up a gate, with a ledge about 2.5 feet above the ground, where (we figured) the younger, more agile, cats could get over it, but the older cat wouldn't be able to navigate. That backfired. My newest trick was moving the food up at window-level (4 feet, maybe) on a box, and showing my cats where to get it, but I'm not convinced that it will be enough to deter the older cat. Any ideas???