Mrs Pegasus Spc122 has actually taken a card back to the Halifax for cancelling, yet the post still keeps coming, do you really think they would take more notice of a returned letter.
From experience, my daughter kept returning mail sent to her address from various companies and banks addressed to a previous occupant, their was another occupant in between. The result a visit from the bailiffs one morning .
Spc122 Ring the Halifax make an appointment with the branch manager, take what post you have now and tell him/her in no uncertain terms what you think of the customer care at their branch and how disturbed you are by the continued delivery of post to your address after your previous visit. Ask him/her to investigate and confirm in writing that this misdelivered mail will cease. If they say it is incorrect to open someone elses mail, tell them that the card could have got into the hands of a criminal and you opened the letter to have it cancelled correctly, having done that you assumed wrongly that the rest of the letters would be junk mail and not statements.