what you are looking for a proxy server....
the proxy server can be controlled to report what sites are accessed by who (check legal department about privacy of the employee, for the who part, if its annonymous it shouldnt be a problem) and then block those sites.
which proxy depends on your knowledge and budget.
a MS sollution is TMG (threat management gateway) but needs some budget for hardware/software.
a free solution could be a linux machine wiht squid proxy server installed. This requires some knowledge about linux/squid.
or look int other third party software.
for example:
http://www.artica.frlike MAC said avast is not a webfiltering tool and thus wont be able to filter the way that you desire.