Do a command-option-r (all three keys at once) boot to Internet Recovery. Once the recovery screen comes up start Disk Utility and select the drive itself at the top (above Macintosh HD) and erase the drive. Then quit Disk Util and click install OS. It will take a while as the 4.7GB OS had to download.
This will get you back to whatever OS it came with, be that Lion or Mt. Lion.