To disable HTML editing, remove following code from templates/admin/page_add.tpl and page_edit.tpl file


Code:
<script language="javascript" type="text/javascript" src="{$baseurl}/js/tiny_mce/tiny_mce.js"></script>

{literal}
<script language="javascript" type="text/javascript">
	tinyMCE.init({
		mode : "textareas",
		theme : "advanced",
		plugins : "table,save,advhr,advimage,advlink,emotions,iespell,insertdatetime,preview,zoom,media,searchreplace,print,contextmenu,paste,directionality,fullscreen",
		theme_advanced_buttons1_add : "fontselect,fontsizeselect",
		theme_advanced_buttons2_add : "separator,preview,forecolor,backcolor,hr",
		theme_advanced_buttons3 : "",
		theme_advanced_toolbar_align : "left",
		theme_advanced_toolbar_location : "top",
		theme_advanced_statusbar_location : "bottom",
		theme_advanced_buttons_location: "top",
		theme_advanced_resizing : true,
		paste_auto_cleanup_on_paste : true,
		paste_convert_headers_to_strong : false,
		paste_strip_class_attributes : "all",
		paste_remove_spans : false,
		paste_remove_styles : false
	});
</script>
{/literal}