Citibank — Harressment from Citibank callers

I am using citibank credit card for the past one year & paying the dues on regularly basis before due date.Still then i've been receiving calls on citibank behalf for their dues followup with no days limit & no time limit - even on sundays & in late evening almost 5-6 calls per day, everytime calling different person.
in a way of threatening me.I ve made a complaint at Citibank, chennai but of no use.Pls help so that i do not face such harressment & gets call only after the due date, when the dues are no paid by me.