I'm afraid I've had to be quite hard-line about this. With student films, threaten to speak to their tutors: they can actually withold any qualification until they make good their promises. It probably won't go that far, but it's useful motivator. As regards low budget films, they probably get some funding- just threaten to tell their backers how unreliable they are. I know it sounds nasty, but if you'd not shown up at the shoot, do you think they'd be nice to you?