This is not the right approach.
Great leaders/strong management must know how to create a healthy work environment and handle grievances.
It's the choice of employees or ex-employees to share the exact feedback of the company they have worked for. This may help the company to look upon to build a strong culture where needed. Also, its a signal to outsiders that if the culture won't fit they won't join.
So, rather than adding the clause that you have mentioned, please try to work on improving the areas that needs attention.
Please accept feedback, good or bad it will help both the individual and company. By adding the clause that you have mentioned, I don't think you will encourage honest feedback from your employees on anything about your company which doesn't work in favor of the company.
Feedback is good, even if it is bad.
This is in addition to what previous learned members have written.
Those who upload bad remarks on the company, they do it by concealing their identity. Bad remarks about the company are given even by the current employees and not just past employees on the forums like mouthshut.com Therefore, if your past employees do the same then how will you catch them?
Secondly, there are negative remarks against the companies like Infosys or Wipro also. Notwithstanding these remarks, they have grown and continue to grow. Therefore, why are you so scared of the negative remarks?
Thiyagu has written about improving the quality of the administration by way of taking feedback etc. You may work on these suggestions.
Then few will exit and fewer still will write adverse about the company.
Company cannot stop ex employees from commenting what they feel abut the company in various forums.
Laws of defamation are there to shield the company if wrong reports are given about the company.
A deeper insight has been shared by learned contributors above. Its true that almost all of the negative reviews posted on social media are anonymous just like your query.
In my opinion these clauses work in a reverse(psychology) manner; this way you are telling employees that they can also post reviews online upon quitting. Good or bad, they will choose by themselves basis their sentiments for the organization.
The idea to incert the clause " any Employee during tenure or even after leaving organization can not write/post any Bad/Negative reviews about the company/Management/any employee on any social formus/Site" is good a good idea. But.........
I do not think this clause would help you in any manner? The employee can be held guilty for this type of action(Misconduct) while on employment.
What will you do ?
Someone come with bad posting against your company/management using all types of media but with solid proof against his claim.
This gives an impression that everything in your company is not in order. The very nature of the situation is commanding you to take defence under such clause. You may think can be good but to be proved futile, when the claims are true and truth.
Instead of going for defence try to eliminate the pot wholes that you all know are existing.