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Since I haven't see their contracts I'm somewhat limited but I doubt very seriously if it is possible to write a contract that completely restricts ex-employees from talking to employee at all. Employment contracts must be "reasonable" and barring any contact between friends is something that courts would look at with a very, very skeptical eye.
The only thing that you can really do, without evidence that the employees are doing something illegal, unethical, or that breaches the agreement, is to have a local lawyer write the employees a letter with veiled threats in it and a reminder about the terms of the agreement. Often that will have the effect that you desire without it being necessary to actually sue. Also, if you sue and you aren't able to prove that the agreement was violated then you not only lose the amount of money you paid your attorney but the judge is also likely to order your company to pay the other side's attorney's fees.
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