Honestly, I would applaud them for the fact that they're doing their work so efficiently. If they're the type of person that gets a task and completes it in a quarter or half of the time expected, I think I made a good hiring decision. Heck, am I actually winning since I'm technically underpaying this person?
I would consider looking into why I'm not challenging this employee. Maybe they need to be paid more or given more difficult tasks. I would talk to them and see if they wanted more, if not, fine they're doing their work and I'm paying them appropriately. Pretty simple really.
What's the point of an employer getting upset about getting what they paid for?