500 Fast Cash Wanting me to pay for a loan that I never got. Internet

I have been getting calls from 500 Fast Cash and they have been calling my work. They keep saying that I took out a $350 loan with them in 2012 and I owe them $650. I keep telling them that I dont know what they are talking about and they told me to get a police report done and fax it to them and I urged them to send me what paperwork they have on me that I agreed to the loan and they cant even show proof of that. Now they are calling my work and it has gone far enough. I can only take harrassment so much and they are at the top of the line before I just flip on them. I mean why bother people for money when your not owed for what you claim they took out. I just want them to leave me alone and stop with the harrassing calls everyday.