That's strange.
If this is the genrefilter, the games must have the genre set as their main genre. It will not block games that have that genre set as a regular tag.
If this is the button for the single tag exclusion, you need to be logged into your Itch account while browsing for the feature to work (has nothing to do with my script). And you need to enter the tag as seen in the url without the prefix. So if the tag is shown as genre-visual-novel in the url, you enter visual-novel. If it is tag-3d-platformer, you enter 3d-platformer.
Both those tag blocking options work on my system, as I just tested. I am on Chrome.