Currently there is the possibility to add filters at the user level. Similar could be added the option to add Global Notifications filters.
A user can add notifications filters from the user profile.
Currently the possibility of adding global filter may be too complex for non-dev Admin users.
A good example of global notifications filter that should be added on standard, would be to add the possibility to exclude XWiki space for a certain group of users. Usually is not wanted to be received notifications from other user profile pages, especially by simple users.