Standard Chartered Bank — negative account balance showing for non maintenance of minimum balance even after multiple attempts telling them to close my account

I had a salary account with standard chartered. I left my job and no longer use that account. Right now my account is showing a negative balance of over rs. 9000. I have emailed the bank several times to close the account but to no avail. They told me to send written instructions to certain address with my signature. I sent them the written instructions to that address but they told me that they lost the written instruction that i sent them. I can't keep sending written instructions over and over again just to close my account that i no longer wish to use. Every month they keep charging me more for non maintenance of minimum balance.