Unfair credit practice

I'm held hostage by this bull**** company . I took a contract for a wifi adsl for my house, the line was left hanging and they left the router on my dustbin and according to them the installation was done , I asked them to send me a signed job card for the work done 6 months down the line I'm still waiting for the signed job card and bare in mind I'm continuously billed for this line that is ineffective to date and this has a negative barring on my payment profile and credit score with the credit bureau. I decided to pay them R4200 in march and cancel the contract to date they continue to bill me for a cancelled contract and I refuse to pay more than what I have paid them . I can't continue to pay for something that's not working