If a large enough number of people do it, they would move elsewhere.

A payment system for such a service might be as simple as the propagandist submitting pro-agenda posts with responses from regular posters showing them that the message has gotten out.

You'd think that those PACs throwing around money would want to know the effectiveness of the propagandists that they hire, but I doubt that they even follow it more than very loosely.
If they did, I think that the issue that you are wondering about here would have already been solved.