Free: Contests & Raffles.
If you get out there at O-dark-thirty and decoys are out with no one in attendance you could always do the old trick of picking them up and setting your decoys. Can lead to confrontation but you are in the right and this can be very fun. If they are going to ruin your hunt by breaking rules / laws at least you can cop some joy by returning the favor.
A good duck spot isn't worth getting shot in the face over.