If postpaid, check your bill regularly.If prepaid, keep an eye on the charges made on your balance.Contract customer support and get them to refund/cancel any new service that was enabled on your account without your approval.This is how you deal with Airtel. You need to stay alert to ensure that they do not overbill you. Because they would try doing it on a regular basis.And Twitter is your friend if you want to get support from them.