To flag a comment to one your posts, click the "Report spam or abuse" icon: the little flag next to the comment. When you do so, the comment is removed for everyone except the person who made it. They still see their comment and will continue to believe it's visible to other people.
This is a great way to not feed trolls. They get neither the satisfaction of being blocked, nor the satisfaction of being responded to. They just feel ignored, which removes all incentive for those seeking to make themselves the center of attention.
When you flag a comment on someone else's post, the opposite happens. It disappears for you, but not for anyone else. You can, of course, privately ask the poster to flag the comment for you, which will make it disappear for everybody.
Regardless of who flags a comment, Google is notified and will investigate, at least theoretically.
Rescuing auto-flagged comments
While we're on the subject of flagging, there are two important facts you need to know: First, Google software also algorithmically flags and removes some comments from view. And second, this software is very bad at deciding whether a comment should be flagged or not. As a result, the majority of posts algorithmically flagged and removed from view are perfectly good, constructive, valuable posts (in my experience). I recommend that as part of your normal comment moderation, you go and rescue the comments unfairly flagged by Google.
Here's how: Between your post and the comments, you'll see a link showing the total number of comments. Clicking on that link might show you another link that says, "Show comments removed as spam" -- but only if there are algorithmically flagged comments for the post. By clicking on that link, you'll see all the flagged and removed comments. And by hovering your mouse pointer over the right side of a comment, you'll get a drop-down menu offering "Restore" and "Block" options. Restore makes it visible again. Go to the next flagged comment and repeat.
When not to respond
There are two circumstances on Google+ where the trolls invariably come rushing in. The first is when you post a popular YouTube video directly. The reason is that Google integrated YouTube and Google+ together. So when you embed a YouTube video into a post (by simply pasting in the URL), that video is also posted on YouTube, a notorious troll hangout. When a YouTube user posts a comment on the YouTube version, that comment shows up on your Google+ post, and vice versa.
My advice is to never embed YouTube videos. It's better to use a screenshot from the video, then link to the YouTube video in the post. If you paste in the video link after adding a screenshot or picture, it won't embed the video, but it will be a live link to the YouTube video.
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