Hi Cathy
It looks like the option to cancel an event in the past was 'fixed' very early on in Totara as it was thought this behaviour was not going to occur
It is possible for an admin to remove all users from the event in the past and send a message to these users.
There is also a seminar notification Seminar booking cancellation that is sent out automatically when a user cancels their booking.
Check the seminar setting 'Allow cancellations' is set to 'At any time'
Regards