Unauthorised transactions by FNB on my account

Fnb debited my account with 175 on the 31 July claiming I had a negative balance during a purchase. My last transaction was on Sunday 28 July If my account is on 0 transactions should decline because I never authorized a negative balance... It does not end there... The following day they debited 125. When I called the call center Zoe could not assist me and the second guy put me on hold for 10 minutes. The bankers on chat not very useful.. I need my money reversed urgently!!!!