We don't know exactly how she convinces the girls to prostitute themselves, I don't think it's blackmail/forcing them, it must be more a kind of persuasion to make them feel that they aren't making a serious mistake and lower their inhibitions.
With Mona's situation, when she said she was out of it and she didn't want to do it anymore, Quinn didn't force her with some kind of blackmail to keep her doing it, she let her go. What she did is threatening her not to open her mouth about the prostitution ring and the drugs but that is an obvious thing that she was going/needed to do.
Nevertheless, that doesn't mean that she bears no responsibility for the prostitution situation: she convinces them to prostitute, gives them a place to do it, runs the whole business and obviously gets money. She has responsibility just like the girls have for choosing to do it.
The only "good" thing that I can get out of this shitty situation is that she doesn't force them and when they want to leave she seems like she lets them go.