Some things I found on a Droid Help Site (but you probably looked there):
-Yeah, there’s really no way to do this. Smartphones are meant to access the internet, that’s what they do. You’d have to completely disable the phone to prevent internet access.
-In theory if you had root access, you could rename the tiwlan_drv.ko to say tiwlan_drv.ko.bak and wifi wouldn’t work at all.