<script>
function blockKey(item){
    if(event.keyCode==9) {
      item.focus();
      space = "    ";
      item.selection=document.selection.createRange();
      item.selection.text=space;
      event.returnValue = false; 
    } 
}
</script>

<textarea id="textField" onkeydown="blockKey(this);"></textarea>