Continious harrasment to move from debit order to debicheck mandate

Receiving numerous phone calls to move my debit order to the debicheck mandate. I have on multiple occasions, I told the operators to add a note to my account to NOT phone me regarding the debicheck mandate as I will continiously decline the option as the debit order was sufficient and have I have not missed a any payment. After maybe around 25 phone calls from them where I told them to stop calling me regarding this matter and one email to ***, they still continue to contact me regarding this matter. I am fed up with this.