Just click on the 'flag as inappropriate' link below every comment. This will report the comment to the Scratch Team so they can review it.
If someone makes a comment on your project that you find offensive, you can either flag the comment as inappropriate so the Scratch Team will review it, or you can choose to remove the comment right away by clicking (x) .
Try to ignore the comment, and not to respond to it. It would only attract more attention to the mean comment. You also shouldn't post in the Scratch Forums about it, it doesn't help getting the comment removed faster and, again, only attracts more attention.
In the forums
If you find a mean post in the forums, just click the "Report" link. It will pass the post to the Community Moderators, so they can review it and take action. Try not to respond to it.
What should I do when I keep receiving mean comments?
When a problem can't be resolved by just flagging one comment, for example when a certain user keeps posting mean comments towards you, use the "Contact Us" link on the bottom every page to contact the Scratch Team. They will look into what's happening, and try to resolve the matter with you personally.