Stop Rating emails
Installation and Configuration
The value you're after is stored in the jrUser datastore. Specifically the jr_jruser_item_key table and the user_jrRating_new_rating_notifications key.
When the value of this is set to 'off' no mail is sent.
So you could make a module to turn it off when new users are created. The default for when this is not set is to send the email, then allow the users to de-activate it if desired.
One possible module idea is to listen for when the new user is created, and add the desired entry then.
Docs: "Events and Listeners"
https://www.jamroom.net/the-jamroom-network/documentation/development/1011/events-and-listeners