How to disable selection of a listbox

	<listbox >
	    <listhead>
		<listheader label="H1" sort="auto" />
		<listheader label="H2" sort="auto" />
	    </listhead>
	    <listitem disabled="true" id="li_1">
		<listcell label="1" />
		<listcell label="2" />
	    </listitem>
	    <listitem>
		<listcell label="3" />
		<listcell label="4" />
	    </listitem>
	    <listitem>
		<listcell label="5" />
		<listcell label="6" />
	    </listitem>
	    <listitem>
		<listcell label="7" />
		<listcell label="8" />
	    </listitem>
	</listbox>
<button label="dis" onClick="li_1.disabled=true"/>
<button label="en" onClick="li_1.disabled=false"/>

<style>
tr.z-list-item-disd *, td.z-list-cell-disd * {
color:black !important;
}
</style>