You don't get to book your JMP interview, you will just be allocated a half-day time slot, I think (unless the non-rural process is different to the rural one). And yes, definitely, cancelling is possible. ETA: also, that reason for cancelling at the last minute is completely justified in my opinion. Yes, someone else may miss out on attending an interview, but you don't know your offer status prior to then.
ETA 2: I'd actually imagine this happens not infrequently during those later round interviews. I had three offers prior to my UNSW interview and so didn't bother attending the interview. The date hadn't been set yet, so that part might be different to JMP but the outcome was similar.