Recording of refunds for an event
Welcome to SE!  There are many reasons why one might refund an event participant payment which would affect how the event registration is recorded.  For example, if you refund because someone cancels you also want to cancel the event registration.  If they have paid but they are a speaker you might refund the payment and want to change their role from 'attendee' to 'speaker'.
Conversely, just because someone cancels does not necessarily mean you want to refund the payment.
What is the reason for the refund?  How do you want their registrations to show now? If they are not attending you could select those registrations and set the status to 'Cancelled'