How to change the default settings for Notifications?
Using Jamroom
You're right, you can't as admin set the system up as:
* The profile has a guestbook, but the profile owner will not be notified when someone writes in the guestbook.
By default the notifications are set to ON. The user can then decide to turn them off.
What your after can be done by a module though, although the question "Why?" does jump to mind.