Try this with a user style addon or even writing it in the userstyles file.
I did not even got a response in the thread I posted the same method a few months ago :'-(
@-moz-document url-prefix("https://itch.io/game-assets") {
div.game_cell:has(a[href*="//username1.itch"] ) { display: none }
div.game_cell:has(a[href*="//username2.itch"] ) { display: none }
}
Yes, that works. If it does not work for you, you copy/pasted it wrong or the user style got not activated properly. Also, you need to replace username1 with the exact username you want to filter out, as seen in the url. It is case sensitive for the lowercase url.
And if you do not use game-assets as starting prefix, but itch.io, you will filter them out everywhere in grid view, even your library and collections.