If you don't care about supporting old browsers, you can use :not() to exclude that element:
.parent:hover span:not(:hover) {
border: 10px solid red;
}
Demo: http://jsfiddle.net/vz9A9/1/
If you do want to support them, the I guess you'll have to either use JavaScript or override the CSS properties again:
.parent span:hover {
border: 10px solid green;
}