Registration closure versus registrant editing

Open Events → Events → select your event → Edit Event → Ticketing.

In event Ticketing → Registration Edits, turn off Allow registrants to edit their initial registrations to restrict the enabled registrant edit flow. For complex events the enabled setting allows adding activities/add-ons and removing free items; paid removals do not trigger self-service refunds. New-registration closing dates are separate. If an unexpected addition occurred, ask Support to review the registration, who made the change, and activity availability at that time. Changing the setting does not establish what caused an earlier addition.