you can't stop rational greed and profit motive. there's no way to prevent users from selling coins to colluding parties intent on attacking the network. it's impossible.

the biggest problem with many POS (or hybrid POS) supply distributions is that early adopters have disproportionate control over the supply. in fact, this element is present in POW networks like bitcoin too. but in bitcoin, it's not a problem re consensus and potential attacks on the network. in a POS system, it is. this is why POS remains unsolved.