You can read this to get some idea. It’s an old thread though, so most of the things might be changed already.
Also, the coding that Lyya mentioned. It's from one of the posts above.
Install Stylish (chrome | firefox ), then go into the settings and start “Writ[ing] a new style”.
Find the user ID of the person you don’t like, and put this in the CSS box:
Then set the website that the style “Applies to”, and save, and you’re done!