With Wi-Fi, the normal practice is this: 1) you can use one of Wi-Fi
securing methods, most typically WPA2, to allow access with a
pass phrase, but this secret phrase is the same for all computers; 2) you can configure
wireless router to allow connection only for computers with known MAC address, but the system administrator has to collect valid MAC addresses and support the list of them stored on the router; every new wireless adapter has to be registered on the router data this way.
Please see:
http://en.wikipedia.org/wiki/WiFi#Securing_methods[
^],
http://en.wikipedia.org/wiki/WPA2[
^],
http://en.wikipedia.org/wiki/MAC_address[
^].
The wired access to the Internet is practically never blocked. If someone gets access to the wire, it is quite apparent, so usually only the authorized people will physically connect. Even if some stranger connects to the Internet, it is no more dangerous to your corporate network that any Internet user on any other location. You should really allow wired access to Internet without any limitations, but your corporate domain(s) should be protected (and usually, it is based on password protection, typically using your domain authentication). It has nothing to do with your question.
If you really want to limit Internet access, this is quite possible, but I don't even want to discuss it, as it would make no sense at all and would be simply mean doing the evil. Sorry.
—SA