You can stop the appointment from being booked in the future without altering any previous information. Any previous appointments of this type will still show in the patient's appointments record. Any appointments booked but not yet completed will still show as this appointment type on the report, but if you open the appointment to edit it you will have to change to a new appointment type.

To remove an appointment type, go to Maintenance, Appointment and then Appointment Types.

Looking in the top section of the screen, find the Appointment Type you would like to delete and press the 'x' button in the Del column.