You can delete a reservation for a single class time by visiting "Classes" & clicking through your schedule to delete the pending reservation.
If a customer has a standing reservation for a class, you can delete that by visiting the customer's profile or going to the Standing Reservations report (All Reports => Standing Reservations).
When you delete a customer's reservation , but sure to let your customers know that their reservations have been changed. They are not notified automatically.
How To BULK Delete Reservations
You can remove many reservations at once by going to Reports, then Pending/Upcoming Reservations.
Use the Date Range functionality to select the range you want.
Use the Select All toggle to choose All the reservations:
Then click Actions, Delete Selected Reservations. Your customers are NOT notified, and pre-paid reservations are not deleted.