That's an entirely solvable problem, and numerous key G+ voices (Robert Scoble and Lauren Weinstein come particularly to mind) have long lambasted Google over the lack of suitable moderation and noise controls.
The effect at G+ was that there was a very narrow sweet spot for intelligent conversation -- you needed a large number (~1000 - 10,000 minimum) of followers to generate real traction, but past 100k, the ability to moderate threads was highly tedious. Lauren was (and is) heavy on the one tool he's got, the ban hammer, blocking people. The reasons often aren't that the individuals in question are doing anything particularly wrong, but that the dynamics given the other people likely to come to the thread will simply go haywire.
Or you end up with accounts such as The Economist whose posts are interesting but comments are invariably almost totally inane.
Google exhibited (and continue to exhibit) a profound lack of understanding of real community.
That carries over as well to G+ "Commnities" which are a complete fucking abortion.
