Sybil attacks are prevented by the technology of the underlying blockchains where we will operate. This is because Pool of Stake will exist in native PoS blockchains (we are not creating our own blockchain). If we take Ethereum as a future blockchain where we will operate (after casper is implemented, of course), the slashing mechanism that cuts the stake from a bad actor that could be attempting to spoof timestamps or create separate identities is the main component of the system to deter Sybil attacks.