Use flags when dealing with events (or any manipulation related to the DOM), beacuse event listener can often behave like asyncronous functions.
$(document).ready(function () {
var tween;
var slide = $('#slide');
var flag = true;
function bodyListen () {
$('body').on('mousewheel.bodyscroll',
function (e, delta, deltaX, deltaY) {
if(flag){
e.preventDefault();
flag = false;
moveit();
}
});
}
function moveit () {
tween = TweenMax.to(slide, 0.8, {
marginLeft: 300,
onComplete: function(){
flag = true;
}
});
}
bodyListen();
});