You could have a delivery tax... if you don't take delivery, you pay the tax of the person who takes delivery .. and if you are a hedger you get to right off the delivery taxes you pay as part of your cost of doing business. It reigns in speculators immediately, but doesn't disturb the functioning of any Market.
The delivery tax could be set by some kind of basis level.
The delivery tax could be set by some kind of basis level.