a {color:#FF0000;} /* Unvisited link */
a:visited {color:#00FF00;} /* Visited link */
a:hover {color:#FF00FF;} /* Mouse over link */
a:active {color:#0000FF;} /* Selected link */
!important has the effect that the property in question cannot be overridden unless another !important is used. It is generally considered bad practice to use !important unless absolutely necessary; however, I can't think of any other way of ‘disabling’ :visited using CSS only.