[joeysteele]

I would want to try to get to know the reasons. Everyone can get fed up of things at times and sometimes people can be pushed into thinking the best thing to do is leave somewhere.
At least talking things through or at least trying to do so at least once gives the chance of some understanding of the problems or compromise ,which I believe in heavily,especially if they are a valued employee.

However if they made repeated threats to quit,then that cannot be tolerated by someone with a business to run really.
So after those attempts are exhausted just tell them to go.