and this is for while you're at work?
if so just put a cheap linksys router between your pc and the network.
unplug your pc from the network, plug in the router, plug your pc into the router.
software firewall I would say zone alarm which should have features to disable the internet at specific times, or turn on ie content advisor and set a password. that will limit internet access and ask for a password if the site isn't on the safe list.
or just break the keyboard over said employees head. people learn by example.

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I got your salvation b*tch

wanna go night night fool.