There are 2 ways to delete entries in the on-call calendar.
On the one hand, you click on an entry in the on-call calendar and delete it by pressing the "Delete" button that appears:
It should be noted here that shifts that have already been started cannot be deleted. These can only be reduced to a minimum by moving them. Secondly, you can use the copy mode to overwrite already scheduled shifts with empty entries.
To do this, open the copy mode and select a range in which no readiness is planned. Then insert this "empty" period from the desired date and set the repetitions to 99, for example, to overwrite all already scheduled entries with empty entries.