Unauthorised deductions by MTN directly from my bank Account

I am a business owner with staff on business/corporate packages. One of the accounts has content charges every month that are being deducted from my business account by MTN without any authorisation from me. This means that in real terms, MTN has surely exceeded any mandate given to it as no authorisation to deduct these amounts.

Is this legal? The contract with MTN does not allow for extra products to be deducted by MTN unless authorised by me.