You cannot disable MGMNT but you can still change the password of it. I am using the beetel 450TC3 and it works.
To change the password follow the steps:
>Login to your modem by typing 192.168.1.1 in your browser. Ask to the Airtel customer care for the username & password.
>Click on Setup.
>Click on WLAN
>Click on Security
>Click on VAP0.
>Choose the encryption type and write in the pre-shered key(password for MGMNT). I use "WPA2(TKIP/AES)" as the encryption type.
>Click on apply changes and voila....
Now try to connect to MGMNT using password 0987654321. It wont work anymore.