How to stop mobile service providers from activating unsolicited value added services?

I have two prepaid mobile connections of rim(cdma) & airtel. Both the service providers activate various value added services at regular intervals without my asking and deducting the charges from my account.This causes embarrassment as balance reduces to zero without my knowledge.Any query to the costumer care is of no use as they say that activation is not possible without my consent. Please advice on my future course of action.