HDFC Bank — credit card cancellation policy

Address: Mumbai City, Maharashtra, 400078

I am trying to cancel my credit card using phone banking but hdfc bank is insisting on give a written application and send it to chennai address.
My concern is if they can issue credit card over phone banking and all verification is done over phone banking then cancellation request shall be executed using phone banking. Why am i supposed to write something for cancellation.
This is just mental harassment carried out by hdfc bank credit card department.