the best way to secure your keep you safe from out siders is a secure newtork set up your router to only let certin mac addressis conect.
As far as keeping others out by not alowing internet use, email and ichat still need to be conected to the internet to work so even if you removed web browsing the computers are still have access to the internet and are still potentualy vonerable.
Unless ou have super secret info on your computers then most likely nobody will cate yo hack into your computers for it. If the info is that important dont conect to the internet at all.