It is still possible to get Function constructor:

function Function() {}
console.log(Object.constructor.constructor); // Function constructor!

Why it works: At the time of writing, the code forbidding the Function constructor looks like this:

Function.prototype.constructor = undefined;

And the Function is a variable reference which can be shadowed by local function declarations. (This works because in JavaScript, function declarations are executed before normal statements. Using a variable here will error on accessing undefined.prototype.)

Thus, putting that statement on the top of your solution will leave the real Function constructor untouched.

To forbid these hacks, a method of obtaining the Function constructor without using any variables is required, such as (function () {}).constructor.