What API are you using? You can turn off the customer email receipt through SIM and AIM by setting x_email_customer to FALSE.
I apologize on the behalf of all members and contributors for the delayed response. Please see the answer to your original request below.
How may I disable merchant email receipts?
You can easily update your account to disallow email notifications on your User Profile page of the Merchant Interface.
1) Login to your Merchant Interface at https://account.authorize.net
2) Click User Profile in the main left side menu.
3) Click Edit Profile Information.
4) Uncheck the Transaction Receipt option.
5) Click Submit.
If you want to stop getting the transaction receipts on every authorized charge, like what soundcommerce mentioned above, you just need to uncheck the Transaction Receipt option. The Batch-Uploaded Transaction Receipt is an e-mail message that is generated when a merchant uploads a batch of transactions to the Authorize.Net system. It specifies the number of transactions that were approved, errored, and declined. If you are not using the batch upload feature then you can just deselect this option.