Deducting on the wrong dates

Capfin always deducts my account on the dates not agreed upon. I informed Mandisa that I received a notification from my bank that Capfin will deduct today. I told Mandisa that the agreed date is 27th of every month and if they deduct today, there will be no money and that will leave my bank account with rejected debit order fees. This is not right and she said Capfin has a right to deduct and trace my account any day since my account is on arreas. This is a violation and above all, it messes my account up. Capfin keeps on doing this regardless of my plea not to.