<script>
var a=false;
function sa() {
    if ( a ) return;
        document.MsgForm.strData1.value="";
        a=true;
}
</script>

<form name="MsgForm:>
<input type="text" name="strData1" maxlength="4" size="8" value="홍길동" onkeydown="sa();" onmousedown="sa();" onfocus="sa();"/>
</form>