First: Sounds like a pretty sucky situation.
Second: Sounds like a self-correcting problem. If the city is starving, it will eventually revolt and kick out the duchess. Then, if the ruler is smart, they will give it to someone else.
Third: If you can't ban, then start using other methods to get her out. Get the other lords to stop selling food to the city. Place heavy fines on the duchess. Send in the assassins. Send the banker to cook the books.
Third is possible but unless I am wrong you are thinking in terms of the old system for #2 because why would the city revolting kick out the duchess, sure it gets them out of Margrave but not Duchess IIRC.
