Well, it seems we have a whole continuum of sins, from venial to mortal:
(1) making a reservation which includes an additional day or two that you probably won't use, to provide flexibility in travel dates. When you're sure you won't use them, you cancel the extra days.
Comment: Hey, I've done this myself! Can it really be so awful? But it *does* temporarily reduce the inventory available to other members. Wasn't this Rich's main point? Maybe it's OK because the intended use, however unlikely, IS for a member. And it is probably the main reason why wait lists work so well.
(2) making a complete reservation (for yourself) which you possibly/probably won't use, just to have it available in case it works out.
Comment: This would appear to still be within the bounds of legitimate use, but is a more serious tie-up of inventory.
(3) making duplicate reservations (for yourself) on the same dates to provide flexibility in room or resort selection.
Comment: Legal but IMNSHO crossing over the boundary into rude and inconsiderate. Uncertainty about travel dates (as in 1 and 2) often involves things you can't control. But room size or resort selection is just a matter of making up your damn mind, and doesn't justify tying up inventory that other members would like to use.
(4) making duplicate reservations for a renter on the same dates to provide flexibility in room or resort selection.
Comment: As in (3), only double strength. This is what was raised in the OP. What is the solution? Not sure that I can think of a cure that isn't worse than the disease; you can't legislate against rudeness. Richyams suggestion (rent only for dues) seems extreme, and would likely create a really nasty black market for rented points, as they would be fewer in number but *really* valuable to those nonmembers able to acquire them.
(5) buying several hundred points for the sole purpose of renting them.
Comment: Should be hung from a meathook. This is what we/DVC should try to legislate against rather than lesser infractions. I recall last spring seeing a number of posts offering e.g. "300 BCV points for rent", obviously points they never intended to use for themselves.
How about restricting renting points only in that you can only rent them *to other members*? Very likely another unworkable idea, but I'll toss it out anyway.