Open a command prompt as an Administrator.
Enter slmgr /upk and wait for this to complete. This will uninstall the current product key from Windows and put it into an unlicensed state.
Enter slmgr /cpky and wait for this to complete. This will remove the product key from the registry if it's still there.
Enter slmgr /rearm and wait for this to complete. This is to reset the Windows activation timers so the new users will be prompted to activate Windows when they put in the key.
This should put the system back to a pre-key state.
Hope this helps you out!