I feel you have titled your query wrongly. You are not looking for 'US Visa Policy'; instead, you are attempting to form a policy to retain employees who have been granted a visa and, in case such an employee leaves you, what the penal action will be. It is clear that your company is trying to mitigate the risk of losing employees who have obtained a visa and are likely to leave for greener pastures.
One part of the policy is already decided by you: that you want the employee to continue with you for a minimum of 18 months from the last day of overseas travel.
The next part of the policy will include the penalty the employee will have to bear in case he/she decides to leave before the completion of 18 months. While deciding the penalty, you will have to think of the right figure so that you can really discourage the employee from leaving and also recover the amount spent on the employee for training, travel abroad, and stay abroad.
The next part of the policy is how to enforce the policy. I feel you have no other option but to have a legal bond executed from the employee at the time of sending them abroad; rather, the employee will be sent abroad only if he/she executes the bond.
Take due care to properly frame and draft the bond, taking proper and practical legal advice so that the bond can be invoked in case of need and will stand the scrutiny of the law of the land and courts.
To further safeguard your interests, you can ask for further sureties and guarantors from the employees' side and get surety/guarantee agreements executed from them.