Thanks for reaching out about this! At this time the only way to delete a guest from an event is to change the original registrant's status to Cancelled. From within the event, click View / manage registrants:
Then search for the original registrant, check his/her box next to the Reg Status column, use the drop down arrows to change the Registration status of who you selected to Cancelled, and click Update:
Then you can manually re-register the original registrant without the guest. Just go back to the main Dashboard and click Register someone:
We do apologize for any inconvenience this causes. I've submitted your feedback to our programmers about having the option to simply remove a guest rather than needing to cancel the entire registration. Please let us know if you have any other questions about your event registrations.