To disable your Authorize.NET Payment method, do the following:
Login to your website control panel.
Click FS Control Panel.
Click Payment Methods.
Click View/Manage Payment Methods
Uncheck the boxs for "Credit Card - VISA", Credit Card - "MasterCard", "Credit Card - American Express" and "Credit Card - Discover".
Click Save Changes.
Authorize.NET and credit card processing is now disabled. Keep in mind you will want to enable another type of payment method, such as PayPal so you can process orders.