HARRASSMENT BY CALL CENTRE AGENT

I noticed that the bank was taking monthly debits off my salary each month which I had not asked them to do. I had never received bank statements from Nedbank and therefore could not budget properly. In February and then March, I noticed that the debits were taken off on different dates so I went online and checked my account. These debits were initially going off on the last day of each month but for the past few months, the date kept changing. In March, they took the payment off the day I got paid and there wasn't enough money in the account. I called to have the debit order stopped immediately! Closed the accounts and made arrangements to pay it off. However, since the middle of March, I have received up to 3 sms's, voice calls and telephone calls on about every 2/3 days. I realise I am in debt and have agreed to pay the account off but do I really have to be subjected to this harrassment? I have put in a complaint today with the bank so I'm awaiting a response. The caller on Friday last week threatened that she was going to hand the account over to lawyers. I was so annoyed! What am I supposed to do if I can''t meet the instalments they want me to pay? Please advise asap. Thank