Credit Intel, unfair treatment with NAC, National Arts Council

I settled an account with them, last year, but after a few weeks, they demanded money again, claiming that I had been handed over to them for the second time, with the same reference, which means, they reopened the account I settled last year. Now they are harassing me with emails and SMS threatening to ruin my credit score. How can I be handed over twice for the same issue that I resolved, that is a **** according to me. They work with The National Arts Council (NAC)