Well, speculators either find an avenue closed to them, or they look elsewhere.
How about this -
Dubai sets up a CFD/Swaps commodity exchange where no physical can be traded, just the numbers. For example, Corn is priced at 700, you go long, I go short at $50 a cent (701-702 = 1 cent).
The market rallies to 710, we both cover. I pay you $500 and this money is guaranteed by the Exchange.
Whoever wins or loses - WE'RE BOTH BACK IN BUSINESS
This is a global market now folks, if one country bans something another is thankful and picks up the torch.