When you open the Recipe Sifter (button under the Top Recipes tab in the red bar), click Diet in the first column, Free Of in the second column, and Lactose-Free in the third column.
That is the easy one. For the rest you have to use Filter By Ingredient and that's less friendly at the moment.
So you would go back to the first column on the left and click Filter By Ingredient. In the next column, type "corn" in the box. Now is the "fun" part. A list of ingredients will show up in the next column. For each one that is really "corn" or a corn product, but not, for example, peppercorns, place your cursor on that ingredient. You will see a green checkmark and a red slashed circle to the right. Green is "include", red is "exclude" . click the red circle and that ingredient will be excluded.
Repeat with soy. Now keep in mind that some of the recipes you are excluding might be useable with an appropriate substitute - a different flour for cornstarch, or a different vegetable for corn, or just omit the soy sauce.
Part of your challenge will be learning which commercial ingredients or products you buy contain these.
We also have a great Special Diets forum where you might find more help, perhaps someone who has the same issues,
http://www.food.com/bb/viewforum.zsp?f=9