I've noticed this as well. I've dealt with it by flat out banning any user or page that has nothing to do with HN. I will flat out never look at it again.
Now granted they could probably care less and I probably never would have looked anyway, but it works for me.
Across 4000-some users? What if it's Richard Branson? Isn't he a plausible user? Only way to fix this is add code to the circle compiler that checks for the G+ ID in public comment field here. That catches on virally, and the data gets cleaned. It's been done before for other HN groupings elsewhere.
Here's the Hacker News Circle, in case you still don't have it in your circles (it's really worth it): https://plus.google.com/106419647632534512037/posts