I hope no one flames me for this but ...
I avoid this problem by charging relatives something. We purchased a year ago and the points were relatively high, so they know we are still paying it off (figuratively, because our break even won't be for a few years and we've explained that). They know they can cancel, but if I can't bank the points or rent them, they don't get the money back. I have made that very clear.
It's unfortunate, but only by taking a little collateral from people do you sometimes get them to understand.