I have been struggling since last year to stop a debit order on my account as the merchant has a mandate on my account and have been advised to call the merchant directly to cancel

I have been struggling since last year to stop a debit order on my account as the merchant has a mandate on my account and have been advised to call the merchant directly to cancel. I have tried to contact the merchant but there's no answer. Each and everytime when i dispute the debit order even though I have explained multiple times to Old Mutual that I cannot get through they are unable to assist. This is utterly ridiculous.