How to delete an occurrences of a group service?

You can delete occurrences from your calendar one-by-one or you can use the group service editor to delete occurrences in a bulk.

From calendar:

When you delete an occurrence of a group service, all participants will be deleted, and they'll receive an automatic email notifying them about the cancellation.

Go to your Admin page.
Open your Calendar.
Click on the occurrence you want to delete.
Click on the “Delete” button at the bottom of the editor.
Optionally, you can send a message to your participants explaining the reason for canceling. The “reason for deleting” message will appear in the cancelation email.
Click on the “Delete” button. After canceling the occurrence, we will send an automated email about the changes to all participants.

From group service editor:

From the group service editor, you can only delete occurrences without participants. If somebody already booked a seat for an occurrences then you can only delete the occurrence from your calendar. In this case, the participants will receive an automatic email informing them about the cancellation.

Go to your Admin page.
Go to Services.
Click on the “Edit” button on the group service where you want to delete occurrences.
In the date and time section, select the occurrences you want to delete using the checkboxes. You can delete multiple occurrences at once.
Click on the “Delete” button in the section toolbar.