If you want to stop using chemical products to prevent weeds, here are three effective solutions:
Prevent weeds: Corn gluten in the early spring is an effective pre-emergent, nontoxic herbicide. You can find it at most garden stores.
Keep it green: The best defense against weed invasion is a lush, actively growing, healthy lawn. The grass is able to cover the yard and choke out weed growth. Use a good fertilizer and water the grass at a rate of one inch of water (about 60 minutes) each week in most climates. The best time to water is between 2am and 7am, so it may be worth investing in a timer.
Killing weeds: Pour boiling water mixed with white vinegar directly on the weeds. An alternative is to buy a lawn torch that attaches to a small propane tank to burn the weeds. CAUTION: Make sure the grass is wet to avoid starting a fire.