TRY:
<script language="javascript">
var strB = "<a href=\"#\">a</a><span>span</span>";
var szB = strB.replace(/(<a[^>]*>)|(<[^>]a>)/g,"");
alert(szB);
</script>
<script language="javascript">
var strB = "<a href=\"#\">a</a><span>span</span>";
var szB = strB.replace(/(<a[^>]*>)|(<[^>]a>)/g,"");
alert(szB);
</script>
<script language="javascript">
var strB = "<a href=\"#\">a</a><span>span</span>";
var szB = strB.replace(/<a[^>]*>[^>]*<[^>]a>/g,"");
alert(szB);
</script>