I don't use the LinkButton component very often, but the few times I have, it has generated a pure hatred for that default roll over button look that always seems extremly bulky. Thankfully removing this little rollover effect/style is very simple, just set skin = null.

My Example:

Full Source Code:
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" width="215" height="92">
    <mx:VBox x="0" y="0" width="100%" height="100%" verticalAlign="middle" horizontalAlign="center" verticalGap="10">
        <mx:LinkButton
            label="Default LinkButton"
            textDecoration="underline"
            color="#1B33E5"
        />
        <mx:LinkButton
            label="LinkButtong With No Roll Over"
            textDecoration="underline"
            color="#1B33E5"
            skin="{null}"
        />
    </mx:VBox> /></mx:Application>