You'll need to go into your registry editor (Start Menu > regedit).
Once you're there, expand as follows:
HKEY_CURRENT_USER > Software > Microsoft > Office > Excel > Addins > Esri Maps for Office
Click on that, and for LoadBehaviour, right click to modify, and change the value to 3.
Repeat the same change for HKEY_CLASSES_ROOT if you need this to happen for all users.
Hope that helps!