If the ticketing in your event is not activated, you can easily delete the participant under "Guest Management" > "Participant" via the red cross behind the participant.
If ticketing is activated for your event, you must first delete the participant's invoice before you can delete the participant as described above. Click on "Edit" (1.) for the participant:
Then you can cancel the Bill under "Billing Details" at the bottom of the page. Now you can delete the participant.
Please note: All tickets that belong to this invoice will now be cancelled.