He can't do anything to claim your ETH but he has to be cooperative in such situations like this so he can address it to their technical department. Well yobit has been popular to be one of the worst exchanges with worst support.
Did you used their support ticket for complaining that issue?
I've been contacting people and support for 9 months now. Read the chat log. Dominik Herzog is one of the people I was told to contact; he is one of the YoBit admins. He is claiming its not even possible to get my ETH out of my ETC wallet which is bullshit because they've done it before and so have other exchanges. This is 10 ETH we're talking about, that's a lot of money.