I've had this problem myself - if the reservation is within the confirmation window it can't be canceled. We originally did this on the theory that because the slot was reserved by a member, other's members couldn't reserve it so if they canceled the day of the reservation it is unlikely another member could rearrange their schedule to use the slot. Basically, the first member had disallowed other members from using it so it's going to cost her a slot even if she cancelled it the day off.
Notwithstanding all of that, I can see the value of at least allowing the admin to cancel the slot for them - and that's what I'll put in place on the next code release which should be early next week. We'll allow the admin to cancel historically as well which will allow you to give her the slot back.
Let me know if you have any questions.
