FA 4.2.0

HTML

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css">
<span class="fa fa-stack fa-lg">
    <i class="fa fa-certificate fa-stack-2x" style="color: green"></i>
    <i class="fa fa-check fa-stack-1x fa-inverse"></i>
</span>

<span class="fa fa-stack fa-lg">
    <i class="fa fa-certificate fa-stack-2x" style="color: red"></i>
    <i class="fa fa-close fa-stack-1x fa-inverse"></i>
</span>

<span class="fa fa-stack fa-lg">
    <i class="fa fa-certificate fa-stack-2x" style="color: gray"></i>
    <i class="fa fa-check fa-stack-1x fa-inverse fa-slash"></i>
</span>

<span class="fa fa-stack fa-lg">
    <i class="fa fa-certificate fa-stack-2x" style="color: gray"></i>
    <i class="fa fa-check fa-stack-1x fa-inverse fa-slash2"></i>
</span>

CSS

.fa-slash:after {
    content: ' ';
    display: block;
    position: absolute;
    top: -0.2em;
    left: .9em;
    border-radius: 2em;
    width: .2em;
    height: 2.3em;
    background-color: red;
    transform: rotateZ(-35deg);
}

.fa-slash2:after {
    content: ' ';
    display: block;
    position: absolute;
    top: 0;
    left: 1em;
    border-radius: 2em;
    width: .2em;
    height: 2em;
    background-color: white;
    box-shadow: -0.1em 0 .1em gray;
    transform: rotateZ(-35deg);
}