I don't understand how derivatives apply in a PoS system. Having 'control' of paper money in a closed system like a brokerage is not the same as actually having that cryptographic currency to stake.
It matters because nothing at stake means you have nothing to lose. If you lose coins but gain at least the value of those coins elsewhere, then you still have nothing at stake.
Nothing at stake is fundamental and can't ever be fixed. That doesn't mean it isn't possible to build a functional or useful currency system around that fact, but it will have some properties that will always differ from proof-of-work (where an actual thing -- energy -- is consumed and can't be "unconsumed" elsewhere in an offsetting trade). People who want to do useful work should stop trying to "fix" or "disprove" nothing-at-stake and accept it as a fact and go from there.
Individuals 'leasing' stake to other individuals though sounds like a big problem to me. And any community that allows such a market to develop is probably making a big mistake since it could greatly lower the cost of an attack.
It can't be prevented unless the market is highly regulated. That's exactly what ArticMine was saying.