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.
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.