Receiving phone calls nonstop every day

I defaulted on a car loan back in 1995,but made arrangements to pay what was owed to stay out of court, then in 2003 I received a bill from cavalry portfolio for 19,000 dollars. at the time I owed on a student loan and assumed it was just that. So the lady from cavalry would call and set up payments for 3 months at a time. Then after a while she just started taking it out every month without my consent. This is when i found out it wasn’t the student loan. I told her right then and there not to take any more money out of my account. I explained to her that I took care of this debt back in 1995. Of course the original company no longer exists, and they expect me to find the proof that I paid this money. I was receiving phone calls nonstop every day for awhile, and then they stopped. Now they are calling again, and they even called me at my place of work last week, which I assumed was against the law to do. The original car loan was for 5500 dollars, after I defaulted they were going to sue me for 3000, and i settled to pay 1200(100 a month) to keep it out of court and settle the debt. They never seem to want to give me any information, as to how I owe this money. They do not show up on my credit reports, but they keep popping up as credit inquiries, in order to knock my credit score down. Please let me know if there is anything I can do to resolve this.