Too many people think that any network or service that is set up by voluntary people to guarantee the privacy and anonimity of their online actions is so idealistic that they do stupid things.
Tor is such a thing. It is a network of anonimization servers set up by unconnected volunteers that have nothing to do one with another. You surf to the Tor server with your Torclient and than you surf the web or send your email and you can even get out or off from another exit point.
The problem is that not all these entry and exitpoints are set up by volunteers. They are set up sometimes by police, copyrightagents and hackers-spies.
So there is no free VPN network set up by freedomloving people around the world.
It is now clear that aside from the childporn you can find there, people have also found millions of official documents and drops with logs and logins. Wikileaks and the researchers that found the Chinese spyring Ghostnet found their official stolen documents one one of these unsecured parts of TOR. For anyone to download or intercept.
So you should block TOR on your network.
You shouldn't use TOR for official business (or commercial business).
You should make it clear that TOR is a NO_USE service for your employees.
You can use it now and than for some free innocent stuff like using a service to send fake emails for someones birthday. But for other stuff ? Or you should encrypt and defend your computer completely before going in this palace of mirrors in which nothing is what is seems like.