The stuff stolen from me got succesfully taken down! (or deleted since the user is actually still active, not sure who did it.) However, the google search results still shows the itch.io page on the first row, so how long does it exactly take it to be fully gone? Or it won’t go away unless I do something about it?
About the prevention thing, it looks like that I can’t really protect my Scratch projects from theft aside from putting watermarks like what redonihunter suggested (even though those are still easy to remove just like what Red Wing did on the bitmap thumbnails)… I was thinking into putting a disclaimer on my games saying that any sorts of theft will result on a DMCA takedown (which I actually filled one yesterday, but got stuck at the paywall), but then Scratch would delete my games because of their “embrace remix culture” (let other users copy your projects) policy.
So this means that this could happen again at any moment and left unnoticed for months (probably my fault for not giving enough supervision to this specific game), but the good thing is that the stolen projects got taken down pretty quickly than I expected! (unless they got deleted by Red wing himself)