It's a fool's errand to try to prevent criticism to that extent. It's mostly harmless and functions as a release valve for people's frustrations and helps to stop them from doing something more extreme than just complaining.

It's also possibly illegal to stop them. Employees in the US have a legal right to talk to each other about their working conditions and employers are not allowed to stop them. Most companies aren't above violating that law, but they want to save that for actual union-busting, not to stop people from sharing memes.