I noticed this post today, and thought it sounded like a nice idea ( http://wordpress.org/support/topic/121489 ) so here you go :)
Report Comments Plugin for WordPress
This plugin provides a way for visitors to report inappropriate comments on a WordPress powered site. It does this by automatically adding a link to each comment, which (after asking for confirmation) notifies the administrator by email. There are several options available, including customization of the text, the email message generated (you can choose to include the post title, url, date, author, etc..), and more.
(Written for WordPress 2.2 - not tested yet in previous versions)
More info and download:
Note: While I tested this plugin quite a bit, it is still just a couple of hours old, so if there are any bugs, etc.. please let me know.