Contact food companies (contact info can be found on food packaging) and tell them you want them to take genetically engineered ingredients out of their products. Tell them you will not purchase their products unless they can provide you with written assurance that their products do not contain genetically engineered ingredients.

Another way to impact food companies is through your local grocer. Please ask the manager of your grocery store for a written statement on their policy on genetically engineered foods. Request that they identify products with genetically modified ingredients, and ask them to take genetically modified foods out of their house brand products.