TV Licence rip-off?

Despite having paid the full amount for my TV licence (I believe) to Hahn Collections I am continuously receiving SMS messages to advise me that I still owe them money. I have had no "proper" communication from them in the form of e-mailed statements advising me of additional charges, nor telephone calls - the lack of calls may be due to the fact that if I receive a telephone call and nobody talks to me then I just block them.

I recently sent them two e-mails to query the additional charges, the first one with attachments showing the SMSs and proof-of-payment, and the second without the attachments (in case their mailing system blocks e-mails with attachments). Surprise, no response - not even a delivery or read receipt.