You can’t remove participants from the group service itself. Instead, you can remove them from the occurrence that they have booked in your calendar.
Make sure that you delete the participant and not the entire occurrence. To delete a participant, use the small “Delete participant” button in the grey box of the participant.
Step-by-step guide:
Go to your Admin page.
Open your Calendar.
Click on the occurrence where you want to delete a participant.
Click on the participant you want to delete.
In the grey box of the participant right below the contact information, click on the small “Delete participant” button.
Optionally, you can send a message to the participant explaining the reason for deleting. The “reason for deleting” message will appear in the cancellation email.
Click on the “Delete” button. After deleting the participant, they will receive an automated cancellation email.