Fake zaps have in principle no difference from fake reviews. It's a bit harder to fake, but at scale there will be services automating it, just like there are services for fake reviews. Wot solves for both zaps and reviews, we solve wot - we get useful signal from any kind of interaction. 

 Fake zaps is harder to solve than Byzantine Generals Problem 

 True. "Web of Not Spam" is necessary to filter Zaps. 

Once we have high signal Zaps though, Ratings become unnecessary.