There are ways to incentivize certain behavior, while disincentivizing other behavior. However, there is not really a way to prevent any specific behavior. Such is the nature of the decentralized world.
I wrote out a bit here on using incentives to encourage voting and participation. While it would not do anything to discourage speculation, nay likely the opposite, it would likely encourage participation - even by speculators, which I view as a win, converting them into participants! Feel free to give it a read and provide feedback: