The only way to clear the list of high scores is to uninstall and reinstall the game. In order to remove it, you can use its uninstaller from the installation directory or Add or Remove Programs feature:
If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Penguin Cold Cash and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Penguin Cold Cash and click on Uninstall.
After that, I suggest you run CCleaner or Glary Utilities to make sure that there are no leftover rogue registry keys in your system. Next, reboot your computer and reinstall the game.