- Quit Aperture
- Then disconnect from internet (WiFi, Ethernet, etc.)
- Launch Aperture again.
- In the Aperture Preferences Web Tab remove MobileMe.
After that you can connect again to to the Internet.
Probably the "Quit" button does not work. Then try to quit using the Terminal to avoid "Force Quit":
Launch the Terminal (from Applications > Utilities) and enter
If this does not work, use "Force Quit".
See here why to avoid force quitting:
If Aperture is unresponsive, how to avoid force quitting?