As someone else already said: You can not give someone access to data while not giving them access to data.
What you can make a hell of a lot more difficult is the ability to get the data out in any other way than inside someone's head.
At the extreme range, allow people to enter and exit the building only naked, changing into work-clothes on the inside that never leave the building. Don't forget cavity searches.
Oh, wait - you were planning to run an office, not a prison? That's gonna make things a little more tricky as human beings tend to be picky about archaic things like dignity.
The non-bullshit answer is basically this: The freaking NSA fucked this one up. If you really think a random collection of hints on /. is going to give you a better shot, you need to be fired.
Update your security policy regularily and monitor compliance. Do a good job. Stop worrying about the Snowdens of this world, because there's like one every decade. But users looking for shortcuts, managers wanting a dial-in connection from home, admins leaving the firewall wide open after a change, developers using test-configurations in live, all these things are happening every day. Worry about them.