Bonitas medical fund's negligence

Due to Bonitas negligence,I was *****ulently debited R15k by the fund. I have been a loyal client of Bonitas medical fund for over 20years and I never expected such incompetence and unprofessional behaviour...to hold me responsible for a supposed mistake of theirs that they have been "under charging" me for the past year. There was no prior communication or transparency regarding the matter,all I got was a slap in the face debit order from my account which resulted in all my other expenses going unpaid.I had to drop everything I was doing for the day to go get the order reversed. I was glad to have it resolved by my bank however I got a notification of suspension of my cover from them. I unwillingly opted for termination as I was informed I had to pay the R15K regardless of it being the fund's mistake. Please help me as a current pensioner to resolve this matter.