Sign of the social times
Here’s the tactful workaround: Create a Google+ circle of people you don’t want to snub, but don’t want to share your life with, either. You can call it anything you want—Frenemies, for example—since Google+ doesn’t expose the names of your circles to others. When an annoying semi-friend adds you on Google+, quickly add them to your Frenemies circle. They’ll get a notice that you’ve added them to a circle. Hurray! They’ll even show up on your profile page as one of the people in your circles. By the time they figure out that they never see a single post from you, maybe they’ll be mature enough to handle it.