Thank you messages are customized on a per button basis. Note that this will only display if your visitor has not received a response from the contact they checked in to see. The iPad assumes that the two way communication serves as validation of their check in and welcome to your facility.
To set up a custom thank you message, go to the buttons tab on your left navigation bar. Click 'visitor buttons' on the right then select 'edit' for the button you wish to customize.
Scroll down to the 'customize your thanks screen' section and add your header and message. Don't forget to save!