At least for me in Chrome v71 (Windows 10), deleting the function does not do anything (it keeps existing). But you can re-assign it to become a no-op function:

function parallaxIT() {
  console.log("inside parallaxIT");
}

// delete
delete window.parallaxIT;
console.log("parallaxIT was 'deleted', does it still show output:");
parallaxIT();

console.log("=====");

// re-assign
window.parallaxIT = function() {}    // no-op function
console.log("parallaxIT was re-assigned, does it still show output:");
parallaxIT();

console.log("=====");