This plugin gives your visitors the possibility to report a comment as inappropriate. After a set threshold is reached, the comment is put into moderation where the moderator can decide whether or not they want to approve the comment. If a comment is approved by a moderator, it will not be auto-moderated again but will still count the amount of reports.
Once the code below has been added to your theme, you can enable comment flagging by heading to Settings > Discussion, and checking the “Allow your visitors to flag a comment as inappropriate” option.
