Help get rid of plain green

I initially borrowed $250 from plain green loans and had paid back nearly $400, then took the offer to borrow $500. They subtracted a little over $100 that I had left to pay on the original loan and gave me about $375. I've paid them over $800 total by now and they still want almost 400 more. From what I've read I should be paid in full and then some by now? They keep trying to ram $100 through my bank account every week which I don't have and it's costing me NSF fees. They also informed me that they are adding $5 a day for every day I'm past due. I've seen on here that they are an illegal lender and I should be able to stop them at this point, I just don't understand how. I've read other people say to revoke their ach authorization, does this work or is it possible? If so, how do I do this? What do I say or send to plain green then? Please help anyone who may know or has done this. I live in ohio if residency makes a difference. Thanks for any replies.