bank charges

Please assist me with FNB account that I settled in the 2019 in 14 May, and now I'm getting calls from sheriffs on behalf of FNB on the account, I settled and requested it to be closed. I need your help with this problem because the account is inaccessible even the app does not allow me, I went to the bank in December 2019 about the account and they said its closed. I'm surprised to hear from the sheriffs.