Legally unless it's stated as a clause in your contract there is nothing you can do about it ....
The old saying you snooze you lose comes to play here. If that employee(s) meant that much to you then you would have taken the time to protect yourself from employee theft.
If you as a businessman didn't take the proper steps to avoiding this situation then shame on you and you need to make some fast changes and move on with life.
Sorry for my bluntness but thats the facts jack.
