Whats the best way to prevent your android smartphone from accessing network data?

I don't want a data plan, i'm happy with pay per use...but i've heard stories of people with their android phones connected to network data without their knowledge. Not until the end of the month when their bill arrives to they realise that some program or feature in their phone has been doing this. Is there an all purpose kill switch or widget that will block this completely and always show me its blocked? Im new to my phone and still feeling around.