This one works for Airtel Punjab not sure if it works for other states and territories. SMS GPRS299 to 121 and it will be activated in 6 hours. To deactivate it SMS DGRPS299 to 121. To get the settings SMS MO to 52567.
The customer care exective I talked to also told me that if I deactivate it in 2-3 days I will just be charged for those days though I am not sure if was right about that.