[eternalban]

Communication is merely one component of the necessary means for coordinated action. Lets push the hysterical notion of OP's "activist" to an extreme and consider the situation:

In this hypothetical scenario, humanity has developed the ability to remotely project thoughts to designated individuals. No technology is involved. Interception of projected thoughts is simply not possible. Plans are made, and then time comes for executing the plan, which must by necessity occur in the physical world.

Per your position, prevention of destructive acts by sociopathic actors in the above hypothetical world is not possible?

The freedom to associate and communicate unhampered by special interests -- corporate or governmental -- is a fundamental requirement of free societies. This fundamental requirement trumps every other consideration. For example, in the above hypothetical world, the society at large still has recourse to other means to detect and prevent destructive action. However in the world that you seem to be proposing, perfectly sane and reasonable actors can be trivially denied from the exercise of the fundamental right of free speech, communication, and association.