Disable Automatic Receipts on Existing Recurring Donations
In the Recurring Contribution series itself you can set Email Receipt to Off; if you have access to mysql you can stop sending CiviCRM Contribution receipts for all your existing Recurring series by setting is_email_receipt = 0 ( in civicrm_contribution_recur )

For the iATS extension we have added an additional setting that will override (a global setting) all individual recurring series.