When ants were getting into my cats' food, they were coming in through a small vent to the outside. I'm always battling ants outside, so I just bought some outside ant bait by Terro (it worked very well by the way). Plus, my girls aren't at all picky, so I just moved their food upstairs.
The mint tea bag suggestion is also good. I actually just bought spearmint and peppermint essential oils to refresh mine. You can actually use the oils all around window sills, etc. too. Essential oils are skin and eye irritants, so you have to watch, but they are very effective otherwise pretty much safe. I use them alot in my basement where the cats don't go much, and I put drops in drawers and in cabinets, etc. I wouldn't do anything like put the drops of spearmint right by the kitty food, because it's also a cat deterrent.
I think somone else had a thread on here saying Bounce dryer sheets are good for ant repellant. Are those kitty safe? Maybe you could put some of those underneath the food area??