Account handed over even after I've settled the account

I have canceled my account with netstar and their legal department emailed an invoice with the amount due. I paid the amount due immediately and sent the pop to netstar. Marcia acknowledged receipt of the pop and she said the amount will be allocated but I still receive calls, emails and sms's from nimble asking me to pay. I called netstar and they said I'm not owing anything and my file will be closed with nimble but again today I received another call from this nimble company asking me to settle my netstar account. Can someone please sort this out ASAP.