How to block a user's deviations from showing up in the thumbnail feeds for "Newest", etc:
Add a single line to your Adblock rules. (Click the Adblock icon, Options, Customize (up the top), Manually edit your filters (press the "Edit" button next to it), type in the following text on a new line:
What this does is tells Adblock to apply the rule to anything on the Deviantart website, it looks for any SPAN tags, then checks to see if anywhere in the href (basically, the URL) entry has the text you specify. The *= is saying match anywhere in the entry, whereas just putting = would make it do a perfect match for the whole entry.
I've no idea how well this works for notification feeds, but for the "Newest", "What's Hot", etc feeds it works a treat. I've had a problem where in the time period I generally keep an eye on the newest deviations in a particular category, there's a flood of art from a particular user. I have nothing against this user, I make no judgements on their art beyond saying it's just not my cup of tea, and there's certainly nothing they're producing that would merit my saying anything about it. In over a decade of using the site, this is the first (and I expect only) time I've ever felt the need to do this, and it's purely because I'd like to see something other than just their art in the Newest feed.
The reason I have adblock installed and running on this site? How about the multiple times they've let ads that play sound through? Or the instance just the other day that prompted me to re-enable for DA, there sat one of those ransomware ads. You know, the ones that tell you to click here to scan your computer, then get you to install the 'scanner', and then get you to pay money to continue accessing your computer. Yep. :/