Disable all users email notifications?

I’ve imported a phpbb forum to Discourse.
I’ve also imported some other messages and it seems that all the imported users are receiving activity emails from the Discourse forum, which I don’t want since I’m still heavily working on my forum and it isn’t opened to public yet.

So I need to disable all email notifications for the users as soon as possible. How can I achieve this?

There is a site setting to disable all email. The importers usually set this for you, or they should anyway.