BILLED FOR A CONTRACT THAT WAS CANCELLED

My Mom, Mrs S S Engelbrecht went to Cell C about 3 to 4 years ago looking at a Blackberry phone on contract. She signed the contract and paid her R100 administration fees. While there she asked why there is so many other Blakberrys lying around there and they responded that they are all Blackberrys that needs to be returned or go for repairs. My mom then just there and then decided she does not want the Blackberry anymore if it is giving so many problems. She there and then informed the sales person to please cancel her contract immediately and refund her her R100, which they have done. Now after 3 to 4 years Cell C has handed her over to Nudept Dept Collection. Even though she has never ever received any account from Cell C telling her that she is owing R200, even though they can't tell her why she is owing them R200. They still keeps on threatening her to blacklist her etc. She went to police station as they requested her to do an affidavit to state that she never ever took out a contract with Cell C - Never the less nothing is going through to Cell C or NuDebt. How can you do this to pensioners of 70 years that does not have 1 cent of debt.