Well, the obvious answer may be to just delete it. However, another approach that I use is to change the Font Style to Strikeout. This will draw a line through the text of the meeting and serve as a reminder that it was in fact scheduled and did not happen. I like to use this for keeping a record of how many times a meeting was rescheduled before it actually happened. To set an appointment font style, merely right click on the appointment with your mouse and select from the popup menu Strikeout from the Font Style menu.
This help article applies to Time & Chaos 7