When a new user account is generated, either by setting it up manually or by importing users from a CSV file, an activation email is sent to each user.
This can be problematic when you want to test a CSV import, since you don't want every user getting an activation email as part of your test.
To stop the emails from being sent, create a Global Setting with the name "admin.user.notice.enable" and the value "false".
When this setting is in place, no new user email activation emails will be sent. Be sure to set the value to "true" (or delete the setting, since true is the default value) when you are done testing.
Note that you must then repeat the import so the activation emails will be generated (or individually click the "send activation email" button for each user).