We had a similar situation around here about 2 years ago.  A girl left the company and started with another and the previous employer thought she stole clients for the new employer.  Went to court because previous employer said she signed a no compete that said she could not for any other search/title company within 100 miles for 2 years nor could she disclose to anyone who their clients were.
She won.  The judge said that what she does as a title examiner is her chosen profession, she has never worked in another field and that was all she knew and the judge could not prevent her from making a living.

Now Virginia is a right to work state if that means anything.

What if you have the employees sign a confidentiality agreement to the effect of they can't talk about who the clients are?