trying to block on each pc is going to be a nightmare of a management headache. you should look into an enterprise internet surfing control monitoring system like either surfcontrol, or an appliance such as secure computing’s SmartFilter.
you could also look at the Barracuda Networks Web Filter.
if your company can’t afford or won’t spend the money to get something like this then look into open source systems such as Untangle, (I have not used this but the site states that it is open source and free).
are there any other users out there that have experience with Untangle? or other free web filtering systems?