This is Deregister control. It's used to unregistered user phone number from reminder group.
Click on Deregister control to add to your call flow and setting up as the below:

• Confirmation: is where you can explain to tell to the caller that caller's phone number will be deregister from system. Click on setting button ( ) next to the confirmation label and set the explanation.
• Reminder group: click on drop down list and choose the reminder group to deregister caller's phone number from that reminder group.

Click button Save to finish.

This screenshot showing that the caller's phone number will be deregistered from reminder group Pregnancy follow up.