Wi-Fi problem of the Samsung Corby Pro Mobile

I have ordered the Corby Pro and will be getting it soon i just have one quick question about it.

The plan i have doesnt include data but i know the phone has wifi. Is there a setting i can use that will make the phone disconnect from the internet if i accidentally exit the wifi zone or get disconnected so i dont get surprised with a huge phone bill at the end of the month because i was browsing without wifi withotu realising it?

Hi,
I have been using this set form year and I have the problem with wifi. It connects easily to the wifi having no security code. But when I try to connect it with secured wifi, even if I put the correct key, it just says authentication failed. My family members use the same wifi on other sets. Please someone help me.
Thanks