Dear Yamini,
If your ex-employee has forged the experience letter and if it were discovered at a later stage, then you may issue an office circular stating that recently you have discovered a case of falsification of the experience letter with your company's details. Forgery is a criminal offence, and whoever does it is liable for legal action. The issue of the circular is expected to create deterrence.
However, the issue of the circular also has limitations. This is because conscientious employees will not do the forgery. On the contrary, unscrupulous employees will remain undeterred and will invent a newer way of forgery.
Thanks,
Dinesh Divekar