In this article, you can learn how to turn off Flow/Process Builder email alerts in MassMailer while you are importing any records temporarily.
Goto Salesforce Setup -> Custom Settings -> Edit Bulk Email Sender by removing the API key and Save.
Keep that API key in a secure place. Once you are done with the import process, go back and enter the API key back.
Test this for any PB for some records before you go with this approach. This may still create outreaches though which will fail so you need to clean those outreaches,
Also, turn off the outreach status email notifications by going to MassMailer Setup -> Account Settings -> Alerts.