A 51% attack on a Proof-of-Stake cryptocurrency is an unrealistic scenario and it's strange to bring this up as a critique against Spectrecoin.
Second that but it's not much unrealistic, in PoS coin if anyone wants to have 51% attack, they need to own more than 50% of the total supply. So, it's possible to have own more than 50% if the market capital of that coin is low.
Theoretically and technically you are of course correct. But it would be like shooting in your own foot: you can do it, but why would you hurt yourself. Firstly, at current prices you would need to invest at least 5m to try to aquire over 50% and it would be questionable whether you can even find that many sellers, but let's assume you do. And then what? As soon as it becomes apparent that somebody has over 50% the price would drop to the bottom. And using the 50% to block certain transactions? Somewhat idiotic attempt since, given the privacy and anonymity features all you could do is blocking random transactions, not transactions linked to real accounts / people. So yeah, a wealthy diabolic maniac who wants to destroy the world can theorically attempt to do that. But so far, greed has always trumped pure evil distructive nature.