Thats really odd, Especially with a BB. Do you use Data Normally? What makes you think you were charged for Data while using Wifi. IF you don't normally use Network Data, Turn it off on the phone itself. It's under network connections.
Wifi will take priority over mobole data so the only way I can see data charges show up is when the wifi is not connected for whatever reason. Happens to me often when I go from inside my place to my backyard.