Cell C are overstepping their boundaries

We had a contract that we did cancel a while a go but still Cell C Middelburg and Cell C call centre keeps calling us up to 3 x a week. Sometimes they even phoned my Matric daughter while attending school. We keep on telling them we did cancel it but they keep on harassing us. Don't they use their system and make notes. They are overstepping boundaries according to the POPI act