Greetings!
Enterprise level accounts can contact their Success manager and have the sending authority disabled for the Default group. All users in the group would inherit this property (unless they were explicitly configured to not inherit this setting from the group, but that's pretty rare.)
Because the setting is set at the group level, and users are inheriting it, any user moved out of the Default group would inherit the setting for the new group they are moved to (enabling Send functionality in this case).
Alternately you can manage this at the user level, either individually, or using the Bulk user edit feature
Editing users directly means that their setting is explicitly set for them, and they will no longer inherit from the group they are in, so they can move between group and their setting will not change.
Hope this helps!
-Scott