How about:
Since you have to login to make a reservation the system has a record of your transactions, so why not make it impossible to make more than one reservation for any given date?
This would mean that these multiple block reservations could not be made.
In order to stop the early reservations with subsequent cancelling (this might not be popular!) increase the cancellation charges, or maybe you could have to pay full price even if you cancel more than say 4-5 in a block.
Even more annoying is the folks who drive out from a city and put a tent or chairs on a non-reservable (usually provincial) site 5-6 days ahead of when they intend to use it, just so they have a site available at weekend, making it unavailable to any other camper. I realise that they will have paid for these unused days. But these selfish people are in effect blocking the use of unused sites.
End of rant!
Trevor