There are a few ideas in this old thread. I'm lucky enough not to have dealt with this issue myself though. One obvious solution is to switch to timed feedings, and make sure all food is stored in ant proof containers. Good luck!
The easiest way to keep them out is to put the cats food bowl into a shallow tray of water so that the ants can't get in. You need to keep the water level topped up and if your cat's a messy eater you'll need to clean the tray after every meal, but it keeps the ants away.