there is no supported mechanism to do this.  One would have to write a small application to get and modify each users mail configuration

I created the one in the answer here https://jazz.net/forum/questions/137079/resolved-not-a-bug-with-user-mail-settings-and-receivemails
to disable emails altogether. But it is the same config data.

not that the system assumes the no data present is the same as the default (this minimizes memory consumption). so beware that changing it for all users will add the config data to the database and to memory consumption.   100 users no big deal, but 10,000 might matter..