we've been working on basically the same problem in the email space for a few decades (legit email vs. mass spam). its a very hard (i think impossible) problem.
Well, it is technologically not that hard to imagine a solution---the problem is social, getting everyone to agree on how to do it, figure out the policy around it, etc. And the situation is getting so bad that maybe it is time for someone (maybe someone reading this thread) to figure it out.