Good find.
I didn't know about this user script, but it can be very useful, and I looked for something like this for a long time.
Just installed the script and it is behaving in similar way like when you ignore member, all his topics look almost the same as hidden boards with this script.
Maybe someone can fork it and make some changes,
I would like to see option to totally hide boards.I quickly looked at the source code of the script, it's actually very simple.
Open the "Ignore threads" script in the Tampermonkey editor. Go to the last line (below '// Style') and add this in a new line:
$('body').append("<style>.ignored_topic{display:none;}</style>");
This should hide the entire ignored topic line. I tested in Firefox browser, and it works.
The only drawback is that the "+" button, which previously allowed you to unignore the thread, is now hidden. The only way to restore the thread is to click on "IGNORED THREADS" in the main menu and manually remove the topic ID from the list.