Metronics alarm system MONI Still billing me long after they stopped monitoring my house Dallas Texas Nationwide

I switched from Metronics years ago but I was still locked in so I continued to pay each month.  I called them and said I wanted to cancel, they called for several months telling me my alarm wasn't responding and reminded them I had cancelled their service.  Now after several years of them charging me I looked at my old contract and my term was up so I called them.  I was told I hadn't sent a letter, so in Aug I sent a letter.  Now it is Dec and I'm still being charged.  The cancellation department says I never sent it.  Now to be more user friendly they will do it by email.  We'll see.