How to add nofollow to drop down

I have a drop down list which filters category ASC and DESC. Google doesn't seem to und开发者_如何学Goerstand that is duplicate content. How can I prevent Google from not following the drop down selections:

<select>
        <select class="sort-by-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;" name="ssort">
        <option onclick="window.location = "http://mysite.com/category1/sort/_post_name-pp-asc/"; return false;" selected="" value="http://mysite.com/category1/sort/_post_name-pp-asc/">Name ASC</option>
        <option onclick="window.location = "http://mysite.com/category1/sort/_post_name-pp-desc/"; return false;" value="http://mysite.com/category1/sort/_post_name-pp-desc/">Name DESC</option>
    </select>

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Pretty sure rel="nofollow" only works within <a> tags.

Don't think it will work with a window.location