The Email Notification can be turned off or the level can be changed using the below steps

1. Go to Salesforce Setup. 

2. Select Email Administration –> Deliverability.

3. Set the Access Level.

No access: Prevents all outbound email to and from users.

System email only: Allows only automatically generated emails, such as new user and password reset emails.

All email: Allows all types of outbound email. Default for new, non-sandbox organizations.

Reference:

https://help.salesforce.com/apex/HTViewHelpDoc?id=emailadmin_deliverability.htm