Without seeing your styles and the way you applied them to the content it's still guessing, but the style Robert supplied should ony set the bottom border. To be sure no other border properties are set, append his example like so:
[B]border: 0 none inherit;[/B]
border-bottom : medium double #00ff00;
The border shorthand property will first clear every border setting; next, the border-bottom shorthand property should only set the bottom border.
Apply these styles in the following manner (just a simple example):
<p>This sentence contains a <span class="zog">double-underlined</span> word.</p>