Anyone can cancel through GigSpace: you, the act, or the venue; and every cancellation records who, why, and how much notice was given. That log is what tells you which acts and venues are actually reliable.
When a gig is cancelled, the payment to the act and the invoice to the venue are voided automatically. No cancellation charge is applied by GigSpace in either direction.
Clearing dates nobody is booked on. On the Booking Calendar, Cancel open dates calls off every date in a range that has no act booked and none asked, in one go: a week you aren't running, say. Choose the range (and one venue, if you like), check the count, and pick a reason. Dates with an act booked or asked, dates with a shortlist in front of the venue, dates waiting on cover and dates a venue requested are left alone. A weekly run carries on after the range, unless the range takes every date it has left: the count tells you when that would end a run. A venue on GigSpace is told once, however many of its dates went.
If an act doesn't turn up at all, mark it as a no-show. That voids the payment to the act, and by default voids the venue's invoice too; you can keep the venue invoice or issue a partial credit if the circumstances call for it.
Marking a no-show is deliberately a considered action with the consequences spelled out, because it's one of the few things in the system that moves real money.