How do you decide when it's time to fire the office troublemaker?

So, all of the management courses I take say “don’t keep trying to turn them into model employees, just fire them”. I thought that was awfully coldblooded, but now that some time has passed, I’m starting to wonder. I personally don’t much care for this person, so I think I’m bending over backwards to avoid making an emotional judgment. When is it time?