If you have backed it up and there's no personal information (credit card numbers, Social Security number, etc.) stored on it, then just create a new admin user. Then login with the new admin user and use System Preferences Users panel to remove the other user you created including all the user files.
If you do have personal data you don't want a hacker to hack, then you'll want to boot from the installer discs, erase the hard drive using the security option (write zeros to the whole drive). Then reinstall OS X fresh.