Is there a way to send out a thank you letter after an appointment without requesting a review or survey?

Yes, you can modify the content of the Review & Survey Request automatic communication in the Campaigns tab. 

To do this, delete the button in the text editor and modify the text to say what you would like it to say. However, the application will still only send out no more than one every 3 months to a specific patient. In other words, if the patient has one appointment this week and receives this communication, but then has another appointment next week, they would not receive the communication again.