Discovery Unauthorised Deductions

I recently joined Discovery Medical Scheme for which my employer pays directly to the medical scheme,Out of nowhere Discovery decided to deduct direct from my personal account,i called them and they promised to refund after 2-3 days,whilst still waiting for refund,boom on the third day they deduct again instead of refund,now this time my account was on minus they deduct from overdraft,i called them again and the promised to refund now double the amount,to make matters worse they will refund only what they deducted but no with interest thats of they will refund