1. There is a debug option in the authorize.net plugin which should be turned off. Please check on that field.
2.
Looking on that page
www.authorize.net/support/merchant/Trans...onse_Reason_Text.htm
the error you have comes from your Login ID:
The transaction request must include the API Login ID associated with the payment gateway account.
If you don't want the activation email, you should turn it off in Joomla. You can do that in the configuration of joomla if you're using joomla 1.5 or in the options of the user manager in joomla 1.6 or 1.7.