Harassment and poor customer service

i used to have an account with woolworths which later was in arrears and handed over.

Jansen and Jansen attorneys contacted me to collect the money.

i paid it up in full and Jansen and Jansen even sent me a letter to show that i settled the account.

later i am now being harassed by MBD to collect additional money - i explain to them time and again that i settled the account with Jansen and Jansen. I even send the proof of the settlement letter. when i send them the settlement letter no one comes back to explain what is happening i just keep getting their annoying sms that i must pay.

Jansen and Jansen tells me when they handed back the files to MBD it was fully paid and owing nothing.