Someone said that they succeeded to keep two open windows and cancel / make a reservation at the same time.
VERY IMPORTANT: I am not sure that it still works. Maybe it was a glitch and fixed since last time that member posted his success .
So before attempting to do what I write below, it's better if you try to book a night and then test the procedure (if you have the points to do so without borrowing). I haven't tried this myself.
Here how to do:
1) open two different Internet browsers (let's say Chrome and Firefox). They must be different because you want to keep the two session separate. Login into member site in both browsers
2) In Chrome look for your reservation
3) In Firefox go to book the new vacation, search dates, room type and resort. In the result page, click "Select this resort" to start the booking process
4) Now the room will be kept for you for a few minutes in order to allow you to enter all data needed. It will not be available to other members during that timeframe. If you don't finish the reservation it will go back to general availability
5) Switch to Chrome and cancel your reservation
6) Switch to Firefox and refresh the page, you'll see the points back in your account. Now you can finish your reservation