FNB trying to **** me !

I opened a credit card at FNB after the called me about 5times to open it . I used it for a few months than decided I wanted to close it . They told me to settle the amount and fill in some forms and itl be all done . 8months letter I get an sms saying I'm owing close to R300 and I do not understand as I've been to about 6 FNBs and they all tell me they will see to the issue and sort it . This is ****ing me off coz FNB likes to take chances! I'm going to drop FNB if thy continue !