Disable email notifications for all Chatter Users via Data Loader

Users can set their own Chatter Email notification preferences
 

See Guidelines for Managing Your Personal Chatter Email Settings.
 

Admins can disable Chatter Notifications for users via Data Loader

Perform an update operation to the User object via the Data Loader.

Set the UserPreferencesDisableAllFeedsEmail field value to TRUE to effectively perform the same action as a user deselecting their 'Receive emails' preference in Chatter Email Settings.
 

1. Export data from the User object to create a file using conditions of your choosing. Ensure you select both the ID and UserPreferencesDisableAllFeedsEmail fields on export. 
2. Create an updated file in Excel containing two columns titled ID and UserPreferencesDisableAllFeedsEmail and populate the ID column with the desired user's unique Salesforce ID.
3. Populate or change the UserPreferencesDisableAllFeedsEmail column's values to "TRUE" for the desired rows and save your changes as a .csv file.
4. Select to perform an Update operation via the Data Loader choosing our newly created or updated csv file from step 2.  Map the ID and UserPreferencesDisableAllFeedsEmail fields and perform the update action.
5. Select a directory to where the error and success files will be saved and click Finish.
    

If you want to disable Chatter email notifications for Community Users, please see Mass Disable Email Notifications for Community Users in a Particular Community. Chatter email notifications can be toggled for community users by performing an update operation to the NetworkMember object using Data Loader.