If a user must be reported, it can be done through clicking the "Report this user" button at the bottom right hand of the "What I'm Doing" section of their user page
Reasons to Report a User
The report user option in Scratch 2.0 below the What I've been doing
section on a Scratch 2.0 profile page.
Users should be reported if their user icon, profile page, About Me, or What I'm Working On is inappropriate, or if they have an inappropriate username. For any other reasons, the user should be reported by contacting the Scratch Team and describing the situation with more detail.
If a user has an inappropriate project or studio then it should be reported through its report button, not the user report button.