500fastcash gave me loan I didnt authorize , Internet

Gave me a loan I didnt authorize. Made a deposit to my checking account of 300.00.I found out about the deposit before they took any money out of my account. I told them I would repay only the 300.00 since I didnt authorize the loan. they said no. They said I would have to repay 390.00. I said no. I closed my account before they could make a withdrawal and opened a new account. They say I owe atotal of 510.00 now. This has been going on for 18 months now. They still call about three times a week. Thank God for caller ID I just dont answer their call. Let them sue me if they want I dont care.