Have to Write a Letter to Cancel Tracking Device

I was called by MiX Telematics Africa for installation of the tracking device on my car. I was on my way to hospital and did not understand and thought they need to check the existing tracking device since I already had another tracking device with a different service provider. They went ahead and installed but now when I request that I need this cancelled as I am not able to afford paying for both tracking devices and I am a pensioner, they say I must write a letter to request this. If there was no letter written to get me to take this tracking device, why do I have to write a letter now. I would like the recording of my calls to hear if there was ever a mention of how I would cancel this if I wanted to and hear if writing a letter was part of it.