If an employee gives notice and you think that you may want them to work for you again in the future, it is better to be professional regarding their departure.  It’s not worth burning the bridge if a few years down the road, you want them back.  It’s better to act professionally throughout the whole situation no matter what, as you never know who they know or how it might affect your business.  If an employee chooses to leave, don’t react in a negative way.  You can be disappointed, you can even be angry, but don’t take your emotions out on them.  It may not be personal – they may be making the decision for reasons that don’t involve you.  And if their reasons do involve you, then maybe you need to take a look in the mirror and uncover what it is that is driving good employees away.  

Burning bridges isn’t always bad, but make sure that you’re only doing it if you’re completely sure that you never want to work with that person again.