I am not familiar with policy on Scratch. Do you even have copyright on stuff you release there?
All user-generated content you submit to Scratch is licensed to and through Scratch under the Creative Commons Attribution-ShareAlike 2.0 license. This allows others to view and remix your content. This license also allows the Scratch Team to display, distribute, and reproduce your content on the Scratch website, through social media channels, and elsewhere. If you do not want to license your content under this license, then do not share it on Scratch.
Please read the license. Anyone can redistribute and "remix" your stuff anywhere, provided the license agreement is followed. Also you are not allowed to restrict that content after the fact or revoke the license.
While it is a policy, the consequences of that policy are not mere policy. It is license stuff. If the copycat had included attribution and what else is described in the license and links to the license everything would have been in order.
Also, did you block that account? If so, maybe unblock it and reload the page...