Clarity's terms of service state that:
"You (as a Seeker) may cancel an Appointment without penalty; provided, however, that you have not already initiated the call with the Expert via the Services. You (as an Expert) may cancel a scheduled Appointment without penalty at any time."
I personally think that if the cancellation is done up to about 3-4 hours before, then it's reasonable not to charge, but anything less than that should come with a partial cancellation fee. I personally spend anywhere from 10-30 minutes preparing for a call in order to promise the most value for the caller. I don't expect to be paid for this time, but if the caller doesn't cancel and just doesn't show, there should be a cancellation fee. Alternatively, each user (including experts) should have 2 'no-shows' after which they get blocked.
If it gives you any comfort, I know that the caller's 'Clarity Score' is negatively effected.