Wow, you've got your hands full at the dinner table!
Cats are carnivores so they don't need all that many veggies in their diet. Change their food around and see what else they would eat(but seeing as they can eat pita bread, they may not be as stubborn as many cats are) and bread's a no-no too as the carbs are bad for your kitties.
I agree with Phoozles. It may just be a habit that they picked up. Could hunting = snatching food out of people's hands? And maybe Elvis learned it from Freda?
Well, I hope you find the solution because it would be terrible if they never lose their title of being a nuisance.