Customers will receive an email receipt when your Transaction Status is "Approved" and the Transaction Type = Charge. This is automatically setup through a Workflow Rule when you install Chargent.
To disable this, you can go under Setup, Process Automation, and Select Workflow Rules. Next to Customer Receipt click "Deactivate". The email will no longer be sent upon each transaction.
To modify the Email Receipt you would want to go under Setup, Email, Classic Email Templates. You can manage and edit the template to include additional merge fields or change the style of the receipt.