Instructions
Go to the main menu of "Pokemon HeartGold" or "Pokemon SoulSilver" on your Nintendo DS.
Choose the "Connect to Pokéwalker" option and wait until you see the "Save, then go for a Stroll" message.
Immediately press the "X," "L" and "Down" buttons on the Nintendo DS simultaneously and hold until you receive a message asking if you wish to erase the Pokewalker.
Pick the "Yes" option and select "Erase Pokéwalker Records" to completely remove the data from the Pokewalker.
Connect the Pokewalker to your Nintendo DS when prompted to finish restarting the device.