In general the idea should be exactly the same as when you send the actual calendar invitation. (Send the email & add the appointment attachment). Now you just need to change the the calender appointment in a way that it will be a cancellation.
I don't actually know whether the above will work (never needed to do this), but if I were you I would first of all check the methods of the CL_APPOINTMENT, if there is something referring to cancellation etc.
Also, you most likely need to change the content of the actual file. For example there is the part METHOD - you should try to use METHOD:CANCEL or something like that. Just try to do some Googling about this file creation part - and don't restrict your self only into SCN (or any SAP related pages), because this is Outlook related functionality, so you might find your cancellation examples everywhere.
Also, as Rick said, do this outside of workflow/SAP first. Send invitation to your colleague, and then cancel the appointment. See how the cancellation (file) looks like at your colleagues Outlook, etc.
Regards,
Karri