I agree, it's very frustrating. If you have rearranged your schedule just go in and cancel the applications that are outstanding. The other thing you can do, depending on your relationship with your scheduler, is to ask to reschedule to a later date if you have the time. Some of the shops on ISS can be rescheduled by the shopper but if you can't do that try reaching out to your contact person. A lot of the shops are put out at the beginning of the month but they have till the end of the month to get them all done. If you don't have time, just decline them.
I do a couple of things to help with that...
I agree with MF Johnston that you are not the "ideal" shopper if they do not assign the shop in a day. Once you have rearranged your schedule, cancel your application to the shop. This is better than cancelling the shop after it is assigned.