`greedy: true`

$(this).droppable( 'disable' );

for ( var i=0; i<=19; i++ ) {
  var images = 'images/slot' + slotNumbers[i] + '.jpg';
$('<div class="placeholder"><div class="img-slot"></div></div>').attr('id', 'slot'+slotNumbers[i]).data( 'slotNumbers', slotNumbers[i] ).appendTo( '#imgSlots' ).droppable( {
      accept: '#images img',
      hoverClass: 'hovered',
      drop: handleDropEvent,
      activate: handleDragEvent
});

function handleDropEvent( event, ui ) {
  var slotNumber = $(this).data( 'slotNumbers' );
  var imgNumber = ui.draggable.data( 'number' );
  ui.draggable.addClass( 'dropped' );
  $(this).droppable( 'disable' );
  ui.draggable.position( { of: $(this), my: 'right top', at: 'right top' } );

  if ( slotNumber == imgNumber ) {
    ui.draggable.addClass( 'img-match' );
    ui.draggable.data("valid", true);

    imgMatch++;
  }  else {
      if(ui.draggable.data("valid")) {
         imgMatch--;
         ui.draggable.data("valid", false);
      }
  }

function handleDragEvent( event, ui ) {
   $(this).droppable( 'enable' );
   if(ui.draggable.data("valid")) {
         imgMatch--;
         $('input[name=Score]').val(imgMatch);
         $('#score h1').text(imgMatch);
         ui.draggable.data("valid", false);
   }
 }

$(this).droppable( 'disable' );

$(this).droppable( 'enable' );

drop: function(event,ui){
    ui.draggable.addClass( 'dropped' ); // taken from your code & I used to change color

   ui.draggable.data('droppedin',$(this)); // associate the container
    $(this).droppable('disable'); // disable the droppable
}

drag: function(event,ui){
       if($(this).data('droppedin')){
           $(this).data('droppedin').droppable('enable'); v// re-enable
           $(this).data('droppedin',null);  // de-associate
           $(this).removeClass( 'dropped' ) // remove class
       }
   }