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.
I understood it clearly with this summary. That guy is just a chat admin so he might be not oriented with that technical issue but if he told this to his teammates that works at technical side. I'm sure it will be solved. There's also a problem with that Dominik, he never took your problem as important as others. Chat mod's/admin must take any issues seriously but he didn't do that to you. Yes, 10 ETH is too much and since last year it's known that yobit is hard to reach if you got issues with them so I'm wondering why you chose to trade there.
Do yobit have other chat admin?