This addon allows you to exclude artworks of specific users from showing up in Recommendations section as well as tag pages by adding said users to a blacklist. Their artworks will be simply hidden. The addon will display a counter to show how many artworks it removed. Additional functions

Exclude the artworks of the author on who's page you are currently on from the Recommendations.
Restore proper uncropped artwork thumbnails, in their original aspect ratio.

Usage instructions:

To add a user to a blacklist, right click a link to a user's profile, which would be in a format like "www.pixiv.net/en/users/<user_ID>", go to "Pixiv User Filter" -> "Add User to Blacklist". Removing a user from blacklist can be done in a similar way, just choose the option "Remove User from Blacklist" instead.
Another way to add/remove users to a blacklist is to edit it directly in the addon's Options page, by manually adding/removing the user ID(s). The format of the list is each user ID on a new line. Click Save button to save the changes.
You can see how many artworks were removed either above the artwork grid to the right of the grid header, or on the addon icon on the toolbar, assuming the addon icon is pinned.
If you also want to remove the artworks of the user who's artwork you are viewing currently from its Recommendations section, check the appropriate checkbox in addon's Options page.
"Restore proper uncropped thumbnails" allows you to uncrop the thumbnails and display them in full, in original aspect ratio. Idea from the "Improve Pixiv thumbnails" userscript.
Export Blacklist button allows you to quickly save the current blacklist as a .txt file, for backup.

Bugs / Issues / Suggestions Can be reported / posted on GitHub