One way to do this could be using a div to cover up the scrollbar. Simply add a div with the same background-color as its container, and place it on top of the scrollbar. I suggest using JavaScript or preferably jQuery to position the div and remember to make the two elements overlapping; this can be done by setting both of their positions to absolute for instance (and their container's position to relative).
Here's a quick example:
https://jsfiddle.net/jffe4sy3/1/
It's not pretty or very generic, but it works in most cases.
HTML:
<select id="select_id" class="select" size="5">
<option value="1" >test1</option>
<option value="2" >test2</option>
<option value="3" SELECTED>test3</option>
<option value="4" >test4</option>
<option value="5" >test5</option>
</select>
<div id="div_id" class="cover"></div>
CSS:
.select{
height:60px; //set this to your max-height (i.e. max-height:400px;)
position:absolute;
}
.cover {
background-color:white;
position:absolute;
border-left:solid grey 1px;
width:16px;
}
JavaScript/jQuery:
$("#div_id").height($("#select_id").height()+2); // the height of the select + 2px to include the border
$("#div_id").css("margin-left", $("#select_id").width()-15); // the width of the select minus the scrollbar
However I suggest you always use the display:none option when applicable! You should only use this solution in the rare case where it isn't.