Just use the 'onDrop' event, if allowed, do your really dropping action. Or use the 'onDragEnter' event, if not allowed, call 'disable' method to disable the dropping on the target area.
$('#dd').droppable({
onDragEnter: function(e, source){
if ($(source).text() == 'Apple'){
$(this).droppable('disable');
}
},
onDrop: function(e, source){
if (...){
$(this).html($(source).html() + ' dropped');
}
}
})