We've been hearing that you've wanted greater control over who can engage on your public posts. You can now follow the easy steps listed below to adjust your settings and create just the right experience for you on Google+:
1) Click G+ Account Settings.
2) Beside 'Who can comment on your public posts' click the drop down menu and choose from among the listed options, or select a custom group of circles or individuals.
She goes on to mention that if someone mentioned you in a post, they will be able to comment even if you they’re not included in the scope you defined.
Giving users even greater control over who can do what with their content is really an extension of this same mentality, and can only be a good thing.