Airtel — subscribe to model mania service without doing anything - prepaid

Without me doing anything at all, i received a message that i subscribed to modelmaniaw service and that five rupees (₹5) were deducted from my account balance. Why am i charged for a service i didn't even know about? And worse, how am i subscribed to a service that i know nothing about? Are you subscribing us yourselves to take money from us without asking? This needs to be resolved as quick as possible. (Ps: i unsubscribed to the service later, though)