Luxture
"We have issued an appointment letter on the company letterhead, and the conditions that were important to us were acknowledged on stamp paper."
Sorry, but you have missed the whole point completely. A company letterhead duly stamped and signed by the authorized signatory is and will always be considered a valid legal document in any court of law and tribunals. So, if you have done what you said you have done, then believe me, it hasn't changed the status quo except for the fact that now you feel mentally reassured. A sense of false security has been created. That's all.
But the issue still remains. Why is it happening? And happening so frequently that you are pressurized to look at ways to ensure they can be forcefully stopped. Why? Why not just look at the root cause and remove the cause to have a happier organization?
Are you or the management even aware of the implications of taking legal action against an employee for this purpose? Very soon, the news will spread, and you will stop getting good or even average candidates to join you. The only people who will join you would be misfits who are either desperate or lack the skills to be employed elsewhere. Yes, this problem will stop, but so will organizational growth.
Every sale has three components: price, delivery, and payment terms. If money is stuck in the market, then you definitely need to look at your order acceptance process. I can understand one or maybe two people leaving you like this, but from the way you have reported this issue, I presume that the number is quite large, and hence high receivables are being reported. However, that's a sales process issue, not a terms and conditions issue.
But to each, his own... Cheers!