600% interest would certainly be considered usury. There are specific laws pertaining to these types of entities and my understanding is that recent legislation has reined in some of their more egregious deceptive business practices. When did you take out this loan? Do you have the paperwork / documents you signed or received at the time you obtained the loan? I'd be happy to review them and look into whether you have a case against them. However, you said they're likely not even licensed in MA - why do you believe that? I would not identify the entity by name on a public post, but feel free to send the detailed info to me offlist - my contact info is available on my avvo.com profile.
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As stated above, these types of loans typically violate the usery laws of Mass. I would suggest calling the Attorney General's consumer compliant line at (617) 727-8400 for some direction. The Attorney General also has consumer mediation services that may help. Do it soon, you really need to free yourself of this situaion and calling the AG's office is a step in the right direction.