<ul>
    <li><input type="checkbox" onchange="click_checkbox(this, 1)"> <span id="t1">Один текст</span></li>
    <li><input type="checkbox" onchange="click_checkbox(this, 2)"> <span id="t2">Другой текст</span></li>
</ul>

<script type="text/javascript">
function click_checkbox(elem, id) {
    document.getElementById('t' + id).style.textDecoration = elem.checked ? 'line-through' : 'none';
}
</script>