How to securely restore new MacBook air after someone else was first admin user
The simple way is to boot to recovery mode and use disk utility to first erase the "Macintosh HD" partition, then start the installer. No need to zero the drive since you won't be recovering any deleted files at this point. 

A verified correct installer of Lion will then get downloaded from the Internet. 

Here is a guide for you...


OSX Recovery
OS X Lion: About Lion Recovery


Once you go through the new boot process to create your preferred admin account and register the Mac to your Apple ID. Just send in whatever registration info you please when prompted.  This will bump any prior registration if it was even sent.