I have heard that some Scratchers don't change much / anything on remixes / forks of Scratch projects and then share this cloned project on Scratch! I think Scratch should create a clear line between nearly cloning a project and remixing / forking the project by changing the code and some colours. And once the line is drawn, the Scratch Team should take down remixes / forks of projects which have barely any difference to the original project.

This is already the case.
The change must be “noticable” and “meaningful”. Say you scribble over the entire project and do nothing else. That, if I recall, would count as an unmeaningful change, even if it's noticable.

A meaningful but unnoticable change would be for example earning more points in some game or fixing a bug, which would have to be said in the Notes and Credits.

The “clear line” is that:

1. Exact copies are not allowed (there is a report reason for this).
   
2. The change cannot be a mere pixel or something unnoticable. If it's only a code-change, it should be described in the Notes and Credits.
   
3. It's gotta be meaningful;
   

I'm not really able to give any other example for how to describe meaningful in another context here.

Exact and near-exact copies are already prohibited on Scratch. The above poster gave great examples of meaningful vs. non-meaningful changes. If you see a project that looks to be an exact copy, please report it (and include the original link in the report).