Harrasment for a long paid up account even though I've sent them my settlement letter

I previously had debt with Cell C, which I settled in June 2023 with CSS Solutions and received a settlement letter. In July 2024, a company called Nimble started calling and borderline harassing me about this debt. They asked for the settlement letter, which I sent them three times. I was told they would close the account, only to still receive calls and statements from them about the Cell C Wi-Fi account. Their consultants are rude, desperate, and money-seeking, causing me emotional distress and anxiety. This matter was dead and buried, and Cell C should stop selling our names to these hyenas that would do anything for a quick buck. If they do not stop calling me, I shall report them to the ombudsman.