Call 'hideColumn' method to hide a column and 'showColumn' to display it again. Just like this:
$('#tt').treegrid('hideColumn','name'); // hide the name column
$('#tt').treegrid('showColumn','name'); // show the name column
To hide the whole treegrid component, get the outer panel of treegrid first and then hide it.
$('#tt').treegrid('getPanel').panel('panel').hide();