My daughters MacBook Air was stolen but has since been returned and she is now denied access. Should she bring it down to her local store.

That might be the most straight forward approach to cleaning up whatever the thief may have installed on the MBA.

Or, if she has a backup from before the theft, and the MBA has Lion or later Mac OS X, you could restart holding the Command and R keys, and then from the drop down Utilities menu choose Disk Utiltiy. Select the internal Macintosh HD and Erase the storage. Then choose Install Mac OS X so you install a clean copy of the operating system. That will restore the machine to a clean machine that you can setup for her as new. If you have a backup of the content you can then restore eveything to the MBA.

I just had my stolen macbook returned. When I turned it on a 4 digit password pops up along with a warning stating that it was stolen. I contacted apple and got the password from them, used it, and started over from mac OS x utilities. Reinstalled osx lion from there and it locked me out again! What would you suggest? The apple IT people got me all fired up