Dear Isher DN,
Prohibiting an employee to work for the competitor during the employment is strictly prohibited and you can insert a clause to this effect in the appointment letter. Nevertheless, once the employee quits employment, he/she is free to choose his/her employer. The employer cannot block the future growth opportunities of the separated employee.
Delhi High Court has given a ruling to this effect in 2016. Click the following link to refer to the link of the post:
https://www.citehr.com/571397-delhi-...ml#post2321989
Thanks,
Dinesh Divekar