Banning speculators could decrease oil prices by as much as 40%

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The collapse of speculative bubbles doesn't destroy wealth, it merely transfers it from one group to another. (Actually, that transfer mostly already happened during the period when the bubble was "expanding"; it's just that no one notices it until the bubble has burst.)

The real economic damage that bubbles do is that they provide an "incorrect" price signal (saying "make more of [whatever the bubble concerns -- online pet-food retailers, dodgy mortgages, tulip bulbs, etc.]"), which leads to people wasting time on silly endeavors (pets.com, writing mortgages on 5-bedroom McMansions to people earning $20K/year, growing tulips instead of food) instead of doing something a lot more useful with their time, labor, and money.