You need to delete the "read-only" event from its original data source.
- For birthdays, delete the birthday information from the Contacts app, and the event will be removed.
- For other external data, such as Gmail or Google Calendar, you need to turn off the relevant settings in Gmail or Google Calendar.
If you cannot delete the event, hide the corresponding calendar using the "Show/Hide Calendars" option.
Thank you for your understanding.