Help with 500 Fast Cash

I've been following alot of threads on here and have took the advice of cancelling my bank account and sending cease/revoke letters. I took out a $350 loan and the payments that i made to them paid off the loan. I live in NJ and by seeing threads on here, 500 Fast Cash is illegal to lend in my state. I've explained numerous times when they call that what i paid was towards the principal despite them telling me it was for their fees.. They are claiming that i owe $455 and everytime they call that's what they start out with, "how would you like to settle this?" and everytime i tell them the same thing, i sent you letters, i paid what i owed, i owe you nothing. I would just like to know what can i do to get them to finally stop calling me. I keep answering only for the simple fact i don't want them to get a no answer and then call my work, i don't need them harassing me or anyone i work with about this. When i do speak with them, as we all know they are rude, and have even hung up on me when i was speaking, so how legit are they? Also i received a call today saying that he was an "officer" i believe, i asked where, he said from Ohio, he said that they have a claim against my social security # and i asked who had the claim and he would give me no information until i took down a supposed file # from him. I asked for his # and i got 1-206-307-8192, i haven't been able to come up with who this might be and don't feeling like calling them and getting into a fight, i'm sure it's 500. If in fact these are illegal, regardless of me applying on internet, which they tell me since i did, i have to abide by their laws, how can i get them to stop contacting me once and for all. Any help would be much appreciated. Thank you in advance.