Debit order reversal on insufficient fund

Unit trust investment debited my account three times without funds on it and my account is sitting on negative balance due to that and they are unable to reverse the funds , I don't want to close the account due to I get my salary on the account and I dont want to close unit trust account . I just want them to reverse the amount they took from my account but they are failing to do . I sent several emails with no luck so far .