Telkom is still charging me for cancelled service

I cancelled my mobile service with Telkom in 2017 and they are still charging me monthly after having send me a cancellation email detailing how my final bill would look. Every time I try get in contact with their agents they send me running from pillar to post. When I call them I am told I shall be called back, when I tweet them I'm not helped and i when i go to the stores they tell me to contact the contact centers because stores do not deal with cancellation matters. My final bill was supposed to be December 2017 and it's now May. I instead get called with promotional offers when I've clearly stated I don't want to be contacted unless it's in relation with said cancellation. I am at my wits end because I've received no help. Does anyone have any clues as to how I can claim my money for the months I've been charged.