Placing exceptions on the freedom of speech does not mean that lies will get silenced. It means that whatever the government wants to censor will get silenced. Because the government will be the one who does the censoring. Or, if the censoring is not done by the government directly - the government will still be the one appointing the organization who does the censoring.

The freedom of speech must be protected - even if it means letting bad agents spread their lies uncensored. Because if you try to give the government the power to censor them, you'll end up with a new Department of Truth led by Alex Jones (who is now unoccupied)