Agree with Batchelorett.
If someone kept taking your food away just as you sat down to dinner what would you do?
The following is advice given by Gill White on the TTL site.
http://ttlntl.co.uk
In the wild, our dogs would work together as a "pack" to get the food they need for survival. They would then protect the food they were eating as their survival depends on it. The domesticated dog is not much different, especially if food resources play an important part in its daily likes. The fastest way to teach this kind of dog to become aggressive around its food bowl, is to try to keep taking it away, to see if you can. You confirm to the dog, that you are a threat around its food and besides if someone gave me my diner and then kept trying to take it away whilst I was in the middle of eating, I, like the dog, would give them a firm slap!
The way to deal with this problem is before mixing the food up attach a "Houseline" to his collar. A Houseline is a long, light weight lead without a loop or handle and can either be purchased or made by yourself. It must not be the lead you would normally use to exercise your dog. Now, with your dog on the Houseline, mix his food up in full view of him and on three separate saucers put something very tasty. For instance, saucer 1 may have some chicken, saucer 2 a sardine and saucer 3 some small chopped up dried liver. Now, holding the end of the houseline, place the dogs ordinary food bowl, containing his normal daily food, onto the floor and allow him to have a few mouthfuls. Using the Houseline, walk away from the food bowl to the other end of the room bringing your dog with you. There is no need to say anything.