Deducting Account without permision

Good day, i have been with FNB for a very long time, i had some troule in the past with my income and could not pay my overdraft, now i have a savings account with fnb for more than 2 years, now sudenly they are deducting R500 a month withour permision, today they deducted R2000 without permision. All fnb says is speak to Legal department. I have been ok with the R500 but now R2000, how can this be sorted out. I have 4 kids to support. Accordignly to fnb they are ALOWED to deduct money from anyones account.