In order to customize the messages your users get when they unsubscribe, you need to go to your settings page from the left menu and follow the steps below :
- From Settings -> go to Main Menu.
- Select the Main Menu item with the title Followers.
- Click customize
You will get an option to edit the default text under When someone unfollows, the bot will say:
Check the screen cast below for all those steps in details.
For more on customizing your menu items, click here.