UNAUTHOTRISED DEBIT

I m writing to you Peter to help me to stop a Company called National,previously operating as Chubb Security from taking the amounts from my ABSA account .I have explained to them many times and they keep saying they will inform their Head Officebut I find myself all the time deducted the amount having to go to the Bank to reverse it.I want them to stop this incompetency behavior. Please assist me.