If you grab the dish and drag it to the refrigerator they won't announce the grab a serving to everyone (or it will be canceled since you moved it). Also, if the sim is full (or over half full, sometimes over 3/4 full) they don't seem to normally grab a serving from the dish they just made, in my experience. I have experimented with this since I have mine prepare dishes to take to a retail store, and if they eat from them it effects their salability.) If you really want to make sure they leave the food alone but it does not spoil quickly, move it into a Lice Cold Freezer. They seem to leave the contents of the Lice Cold Freezer alone when on the residential lots.