Change "Show less of" to "Show none of" and make it matter.
I have clicked the "Show less of" on Hello Giggles, Huff Post, LA Times and many many others, yet those are still the "sources" that fill my page. I don't want to see these propaganda pushing nonsense sites. Yesterday, as I scrolled down yahoo "news", the top 17 "sources" were all sites that I had already asked to see less of. I understand that yahoo has their biased agenda, as evidenced by the removal of the comment section, but it's frustrating to have a page full of hyperbolic garbage.