Make sure to stop this behavior before it stops being harmless! Heelers are named for that exact behavior - they will "heel" the livestock they herd, biting the ankles and feet. If they cannot herd livestock, they will "heel" you! There are several ways you can correct this problem, and some good suggestions have been made. You have to discipline a heeler more strictly than most dogs, as they are very strong-willed animals. Whatever method you choose, be consistent, and I'm sure you will be able to fix your problem before it gets worse. Good luck!