Purchase some diatomaceous earth at your local organic garden store. It is a fine white powder, and will kill anything that crawls over it. Made of fossilized sea shells, it acts like a field of razor blades, and is completely organic. It is best applied after watering as water washes it away. A side benefit is that it actually aerates your soil as well.You do want to wear a dust mask when applying it however.....not good to breath.
another great method as a preventative is to make garlic tea. Chop a whole clove of garlic. Place the garlic in a pot of water, and bring to a boil. Once boiling remove from heat and let cool. When room temperature, strain into a squirt bottle. You now have an amazingly powerful insecticide and repellent. It works for pests of all types.