The CSS below can be added to your child theme's stylesheet (CSS file) or via a plugin like Reaktiv CSS Builder or Simple Custom CSS.
/* Hide the copy attendee checkbox option during registration checkout */
div#spco-copy-attendee-dv { display: none }
References:
https://eventespresso.com/topic/remove-copy-information-to-all-users/
https://eventespresso.com/topic/how-to-suppress-note-re-applying-attendee-questions-to-more-than-attendee/
https://eventespresso.com/topic/remove-option-to-use-primary-registrant-info-for-all-other-attendees/
https://eventespresso.com/topic/multiple-registration-editing-registrant-names/
https://eventespresso.com/topic/editing-multiple-ticket-purchases/
https://eventespresso.com/topic/remove-the-copy-options-for-addl-attendees/