Yeah just unset the variable. You don't need to destroy the whole session just the value?
unset($_SESSION['CART_ITEMS']);
Check my syntax. Pretty sure that's it. Or you could just re-write the session variable. Like:
$_SESSION["CART_ITEMS"] = "blank";
That would for sure erase the values and put that one in. I don't think that you could do:
$_SESSION["CART_ITEMS"] = "";
Just try a few things like that. Don't use session_destroy() unless you have to. :-D
Chad