Hi Richard,
I've never had a good experience with Yext. As far as I know, they may even create these nuisance listings to drum up more business for themselves - stay clear of Yext is my stance.
Unfortunately, there is no easy process for listing removals when they are involved. You need to send in a message to them directly. Provide a link to the listing, explain that Yahoo! is directing you to the Yext database for removal, and get a verified listing up ASAP to outrank the unclaimed listing.
Then you have to wait 4-6 weeks to even see if it is removed. PITA. You can hangout on this page and wait for one of their operators to come online to speak with directly, that may get you faster and more direct results.