What is the best way to publicize the existence of a fraudulent journal?
Send it to one of the successors to Beall's list of predatory journals. Their websites: 


Cabell’s whitelist/blacklist: commercial, subscription required for access, but accepts listings
Stop Predatory Journals: open-access, crowdsourced via Github, successor to the now-defunct ScholarlyOA site
Beall's original list: now being maintained by an anonymous European postdoc