I belive that (if profitable) miners/groups interrested could rape (think of it as a hack) the protocoll and make PoS useless.
You can't ban PoW-PoW-PoW since we don't have that many PoS, we're only talking about banning POS-POS


If significant (<50%) of miners could agree f.e. on not accepting a small PoS block and only working on top of big PoS and PoW blocks. Just because the protocoll says that you should always build on top of the longest chain doesn't mean it can force you to always do.


How miners could profitable ignore small PoS blocks:

1)New unofficial rule: Don't build on small PoS blocks or better ask for a fixedPoS-fee (no fork or official announcement)
2)A new small PoS apears, but miners rather continue their work on a block where a lot of work has been done already.

No PoS-block could be mined on top of the small PoS so they don't matter at all. Only PoW matters and the honest network would have to be stronger than the unethical network that has already done a lot of work. Beeing unethical gives more rewards since rewards on PoW>>>>>PoS.

Big wallests will always be able to protect there PoS-blocks with a fixed fee and most others won't be able to pay such fee.

--> PoS useless.

The argument ("but then they will have to fight against honest PoS and PoW") doesn't work either. (Pooling PoS possible)

EDIT:red stuff