Airtel - Regarding international roaming bill

I was on a trip to the Maldives in Feb, Airtel does not have any smart package for the Maldives. It was fine as the place where I was staying has wifi connectivity. When I was vacationing there, I got a message of ***** bill amount which shocked me. This was due to mobile consumption. Without my knowledge, they activated international roaming and also lifted the max bill limit amount which was Rs ****. I tried to connect to Airtel telling them I do not need international roaming services and please disable this service but the message was not being sent. When I came back to India, I got to know that the amount is ** thousand. How can they charge so much to the customer? How can they activate international roaming without my approval? How can they lift max bill limit without telling me? Please help, I am not in a position to pay this much bill and no one is helping me.