How can I prevent users from creating appointments?

If you don't want users to create appointments, you have a couple of options:

• Remove the Create Activity (ZMM_CREATE_ACTIVITY_APPOINTMENT_PRIV) privilege from the user's role. Note this action also removes the ability to create other types of activities like tasks. See: How do I edit job or abstract roles?
• Disable the Create Appointment smart action. See: How can I hide a smart action from the Actions menu in Workspace objects?