Rewards manipulation or denying the consecutive PoS/PoW blocks is not a solution.
Disallowing consecutve PoS blocks seems pretty good at this point... why hasn't Novacoin PPCoin done it? I think everyone would agree that if it works, it would be a better solution than centralized check-pointing.
NovaCoin resolved this issue very very long time ago using a complex of measures like dynamic RoI and variable block trust concept.