Endless harassment on account that is not in arrears or handed over

I keep on being called endlessly by Creitworx/Nimble for an account that is up to date. I have confirmed this with your client and yet you the only place who seemingly does not get the message! I have sent numerous emails and have told their call centre agents so many times but yet they keep calling.

I now ignore their calls all the time but they keep calling.

What must happen so you stop calling for an account that is up to date and was never handed over?