How to exclude particular class name in CSS selector?

One way is to use the multiple class selector (no space as that is the descendant selector):

.reMode_hover:not(.reMode_selected):hover 
{
   background-color: #f0ac00;
}
<a href="" title="Design" class="reMode_design  reMode_hover">
    <span>Design</span>
</a>

<a href="" title="Design" 
 class="reMode_design  reMode_hover reMode_selected">
    <span>Design</span>
</a>

In modern browsers you can do:

.reMode_hover:not(.reMode_selected):hover{}

Consult http://caniuse.com/css-sel3 for compatibility information.