JavaScript Shell

This will make the language completely unusable.

clear(this);

Isn't it nice how JavaScript has such a nice function to destroy itself?

This is pretty simple, the clear function completely empty an object. this refers to the global object clearing out everything including constructors and functions.

Because this clears everything, doing anything, even defining a literal will throw an error, making the language completely useless: _{*REPL environment not required.}