Unhappy client

I joined car track in 2020 around December. Everything went well , then I decided to change the car in which they installed their tracker so I nformed them about my intentions. They said they will come and deinstall the unit and keep it until the new car arrives and I won't be paying my premiums until such time. They indeed deinstalled the unit as promised. The new car arrived two months later, I informed them and asked them to come reinstall the unit, they kept promising the whole 2022, until now I've been waiting for them to come. I paid until I got tired of paying for the service I'm not offered. When I stopped paying they harrased me with numerous calls, SMSs and sometimes debit as much as they want as they argue that my account is in arrears. According to them I hav to keep paying for as long as I subscribed to them regardles that they dont fullfil their promises. Now they threaten to list may name as a bad payer. What can I do. I told them I will only pay when they have reinstalled the unit not otherwise.