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WhiskeyWolf
There are several ways, really. In the Sperado's case, the Marquess could either break her mind (shouldn't be too hard, though he'd risk her going berserk), cast a mind control spell (or something to that effect), or cast a spell that gives her immesurable pain if she doesn't follow orders. He could also probably use that chain that was showed in previous chapters (we're not sure yet what it does, but I reckon it can control her).
As for how the Salvatores can use her as a weapon... Well, the Duchess wouldn't use the methods mentioned above because she cares about the cinnamon roll, so she'll probably just raise the girl as someone loyal to the family and teach her how to use her powers. She wouldn't be a weapon, but a "knight".