Traders do provide a useful service, they allow producers of commodities to hedge the risk. For example, lets say you are a framer, you grow corn, and you want to be able to lock in the current price of the crop so you sell a future contract to a trader. The trader buys the contract since he or she is willingly to assume the risk of the farmer for a possible profit.
Without the ability to hedge risk, the farmer may not produce as much crop since he or she is afraid of over supply and that the price they will receive will not support their family. So then there is not enough corn being produced, and prices are then higher for the consumers.
Instead of a trading tax, the normal income tax works, when government interferes in business, they cause more harm then good.