Corroborated by whom? The King of the Internet? Health code violations are investigated by local officials hired for that purpose. For all I know, no such officials have even heard of this kefuffle, but they certainly don't care about what some guy posted on Yelp. Even if a health inspector made an extra inspection on the basis of this, it isn't a hardship to be inspected, so no one can complain.

Assuming you are or know the owner of this pizza dive, no one is "harassing" you by posting a review of your crappy restaurant. If any particular review contains statements which are propositions of fact (not opinion!) and if you can prove those propositions are false (not merely if the reviewer can't prove them true), then you might have a civil case for defamation/libel/whatever.

If you're a USA citizen, this shouldn't be news to you. We have something called the First Amendment, and this is how it works.