The follow steps will guide you through the process of disabling/enabling the Read receipt function
1. Click UserCP from the Navigation Bar a the top of the Forum
2. Click Edit Options
3. Scroll down to the box entitled "Messaging & Notification"
4. Within the "Messaging & Notification" under the "Private Messaging" heading you will see the following text and tick box.
You have the choice whether or not to request a Read PM Receipt when you send a private message to someone by default, or to deactivate the feature. You can always activate it each time you send a private message, in the Additional Options area.
Always Request Read PM Receipts by Default
5. Tick/untick the box for "Always Request Read PM Receipts by Default"
6. Scroll to the bottom and click "Save Changes"