Airtel — to deactivate my postpaid airtel plan

Hello,
I tried to deactivate my airtel postpaid plan but there is no option available for the same. I have not even used my sim card and have got a bill of rupees 450 but my plan was of 399. Same problem happened last month as well and i got a bill of 515. Try to help me with the same because it's not right to pay for things you don't even use. This will indirectly affect you and I will have to change my service provider. Please deactivate it as soon as possible because i won't be paying the next months bill even if i get it.
Thankyou!