Can't be done online. Visit the nearest CSC, take a photocopy of your bank passbook's front page (or whatever the page where the account number and IFSC code are visible), and fill the disconnection form. The refund, if any, will be sent to your bank account. If your deposit is higher than the bill amount (billed and/or unbilled), then you'll be asked to pay the difference. You may also want to take your telephone instrument or the modem, if they are provided by BSNL and if they are not free. If you are in doubt, take all the above mentioned things. That'd save another visit to CSC.