Events can't be marked complete, only tasks.  If you want to cancel an event you would need to delete it.

You can however choose to have past events show in Informant as half transparent, in italics or both by going to Settings>Appearance>Items Style.

Just as a suggestion - If you must keep the cancelled event in informant and don't want to delete it completely, you can try changing the title of the event so that there is something like an "X" as the first letter - or create a new calendar called "Cancelled Events" or something like that and re-assign the event to that calendar so its separate from your active events.