Dear alamarhaulage,
I would like to help you come up a solution for this. First of all, since zapier is used which is quite unconventional to trigger a calendar event, it is best to reuse zapier to cancel the event to avoid complications in the app. My first question is, why not make another form and list all events allowing users to cancel it, send cancels as form responses and eventually reuse zapier to actually cancel the calendar event ? Please let me know if you need help.
Sincerely,
slscripters