Top photo: My standard food setup caused a reflection on the top of my soup. 
Bottom Photos: A piece of foam board placed behind the setup removed the unwanted reflections.

One of the simplest tools I use with my food photography is white foam board. You can get this stuff at craft stores and most stores that have a “crafts” section in their store for $1-2, depending on the size. I use 20″x30″ pieces, mostly for fill light, but on occasion I need to cut out reflections and cut down the amount of lights in my backgrounds with it as well, so to do that I just place one directly behind the setup.

Now, doing this will reduce the amount of light you have in your scene, which is why I almost always use a tripod for my food images. There is still light coming in from above which gives me the quality of light I like to create with my food photography.

BTW, that giant diffuser in the background is a 4-ft by 6-ft Lastolite Panelight Collapsible Diffuser, my new FAVORITE tool for diffusing light with my food photography.