I get someone's elses account in the mail

I keep on getting someone else's mail at my address for years. I have sent this back to P O Box 470 Cresta 2118 so many times now with red pen , not my account, please change your records. No one at MTN checks or makes any effort to stop this it is so annoying and worst of all that person is not getting their account. Will you please get someone to change this billing account