Currently, Scheduler has a capability setting "mod/scheduler:disengage" which controls whether students can drop their appointments. By default, all students have this capability and they see a button "Drop my appointments" in the booking screen. However, admins can change the configuration and remove this capability from students (I'm not sure how often they do that in practice). In that case, students can't drop their appointments once they've booked a slot. They can only change their booking, dropping one appointment and booking another one at the same time.
This seemed to make sense when students were only allowed to book one slot at a time. But since Scheduler 2.7, students can be allowed to make more than one booking, and then the concept of "changing the booking" is more complex (and in fact Scheduler will always allow students to drop their appointments in this configuration).
With further feature requests (e.g., paging in student view, CONTRIB-5194), things will become even more complicated in this respect - which makes me wonder whether the feature to deny students "dropping" (rather than changing) their appointments is really worthwhile.
In other words, I'm planning to remove the setting in future versions. Students would then always be allowed to drop their appointments, unless the appointments are within the "guard time".
However, if you still require this feature, please reply here, and explain why you think it's required.
Thanks
Henning