Looks like you've got several errors in there, most notably using ==
when you probably meant =
. Instead, add an event listener and make sure the assignment works:
var box = document.querySelector('#no_email');
box.addEventListener('change', function no_email_confirm() {
if (this.checked == false) {
return true;
} else {
var confirmation= confirm("This means that the VENDOR will NOT RECEIVE ANY communication!!!!");
if (confirmation)
return true;
else
box.checked = false;
}
});