Though it may not suit your requirements to prevent access to anything on the phone, the apps "Protector" and "App Protector" provide more discriminating password protection by locking individual apps. It's an excellent solution to lock access to email, messages, or the gallery, for example.
Protector Android App Review by AndroidTapp.com | Android Tapp. Android App Reviews
I'll have to look into that. Somewhere someone here was looking to block wifi access for their kids. The discussion quickly let her know the cell towers granted access to the same sort of webpages. This could lock out the browser entirely though.