Canceling a calendar invite

You can't change the status to cancelled within a Marketo-managed calendar token -- but If they've already added the appointment to their calendar, then changing the token on Marketo's servers would have no effect anyway.

However, you can add the event to your calendar, cancel it, and save a cancelled version of the ICS.  Then upload the cancelled version as an asset and send it out to either those people who clicked the ICS or to all users who received the invite.  Obviously, the latter is not something you'll enjoy doing, but as long as you leave the announcement out there without cancelling it deliberately -- not just the ICS, I mean the announcement in the email itself -- there may be confusion.  The choice is yours on that front.

In future I suggest linking to a manually uploaded ICS from the start so you're not so tightly coupled.

I like Sanford's idea, but I have a better one for you:

Close the reg page by deactivating it and turn off the reg campaign.

Then send a cancellation note to those who did Register. Then de-register them from the Program.

Then reschedule everything and send again with a NEW program and webinar.