OK, how about alligators on the AZ border and flaming moat on the Texas border? (I really like flaming moats).
Ok, machine gun nests and man eating dogs in California, Alligators in Arizona, hot buffalo wings in New Mexico, and flaming moats in Texas. That should seal it. Oops. Canada? How about an acid filled moat between Canada and the US? Will automatically replenish every time it rains.