I don't think you understood his suggestion. "Banning" a site would be local to your signed-in google account, not a global ban from results (which would indeed suffer the backfire you mentioned.)
You are totally correct. I completely missed that. Maybe I should drink a bit of coffee and wake up ;)
Although... I have a suspicion that at some point it would effect non-logged in users. Many logged in users banning a site is a signal that may effect the global results. Maybe in the same way marking spam in Gmail...