The governing body confirmed the Sharks would be slugged $30,000 and the Sea Eagles $20,000 after Cronulla coach Shane Flanagan and Manly's Trent Barrett both lashed the whistleblowers after their week-one finals exits.
Both clubs appealed their penalties after they were handed down a fortnight ago, however NRL head of football Brian Canavan said neither would be granted reductions in their fines.
"Given the seriousness of the breaches - and the need for the integrity of the game to be maintained - it was decided that the penalties should stand," Canavan said.
In an extraordinary rant after his side's elimination-final defeat to North Queensland, Flanagan brought a list of 10 grievances to his post-game media conference.
He labelled the referees disgraceful and the NRL took a dim view of his comments after he doubled down in an interview on the club's website.
The club also published an article stating the referees had contributed to falling crowds.
Flanagan sought a downgrade of the fine last week when he penned an open letter to fans in which he said: "I owe you an apology. I owe the game an apology."
Barrett urged NRL referees boss Tony Archer to confront his shattered players after their finals loss to Penrith, claiming the whistleblowers cost them the game.
Flanagan and Barrett's comments prompted NRL CEO Todd Greenberg to draw a line in the sand and ordered the pair to "grow up".
Photo: PHOTOSPORT. Caption: Shane Flanagan Cronulla coach