ALTECH NETSTAR - INCORRECTLY HANDED CUSTOMER OVER

Netstar terminated my account with them in December of 2023. I made my last payment to them on 20 December 2023. Now, they have handed me over to a company called Nimble Group / Norman, Bisset & Associates. I have send several e-mails with the required proof of payment as well as other relevant mails with requests for this matter to be investigated, resolved and for my name to be removed from their calling list as I am not indebted to Netstar anymore but have had little success. To date I am still receiving calls harassing me for this money. Mind you, the amount has increased considerably since then due to interest being added. Kindly advise my next course of action?

Thank you.