When you are working on tickets, we extend an option to close the ticket without an email notification sent out for this.

This can be achieved by clicking on the "closed" button inside a ticket (details page) or on the list view which is basically "shift+close" and a notification would not be sent in this case. 

When you are completely affirmative that you do not want to send this out at all for any of the tickets, kindly navigate to 

Admin > Workflows > Email Notifications > Requester Notifications > Turn OFF "Agent Closes the Ticket."