This is a feature request rather than a support ticket.
I have the plugin configured to allow clients to cancel appointments using the link in their confirmation email as it's sometimes useful for them if they need to rearrange etc but I find some people will cancel a booking right at the last minute, not leaving enough time for us to get another client booked in. I think if the client had to speak to someone to cancel then they would probably keep the appointment, but because they don't have to do anything except click the link they don't worry about letting us down at short notice.
My ideal solution would be to have an option in Appointments+ to set the minimum number of hours which must be remaining for the client to be able to cancel their appointment using the link. Eg setting 48 means that the client can cancel using the link any time up until 2 days before their appointment.
If they tried to cancel after this time then they would be redirected to a different page to the normal cancellation page, where I could add a message saying they need to call to cancel or whatever.