Debit order

I have 4 months with cartrack but their service is very poor, they deduct money into my account when it's the date we agreed on, my agreement with them is to deduct month end which is something they do, during the month like on the 5th or 7th they deduct another money. Today it's the 7th they deducted without agreement....will I be wrong if I reverse the debit order or stop debit order?