You will need to set the property in the client side to disable editing and deleting completely. In order to achieve that, you will need to invoke JS function in InitializeLayout server side event. Here is the updated code snippet. For InitializeLayout server side event:

protected void _webGrid_InitializeLayout(object sender, ISNet.WebUI.WebGrid.LayoutEventArgs e)
{
    short fiscalYear = short.Parse(_fiscalYearList.SelectedValue);
    if (fiscalYear == 2009)
    {
        e.Layout.AllowAddNew = AddNew.Yes;
        e.Layout.AllowEdit = Edit.Yes;
        e.Layout.AllowDelete = Delete.Yes;

        _webGrid.ClientAction.InvokeScript("setWebGridAccess('write')");
    }
    else
    {
        e.Layout.AllowAddNew = AddNew.No;
        e.Layout.AllowEdit = Edit.No;
        e.Layout.AllowDelete = Delete.No;

        _webGrid.ClientAction.InvokeScript("setWebGridAccess('read')");
    }
}

Added JS function:

function setWebGridAccess(type) {

    var wg = ISGetObject('_webGrid');

    if (type == "read") {
        wg.RootTable.AllowEdit = "no";
        wg.RootTable.AllowDelete = "no";
        wg.RootTable.AllowAddNew = "no";
    }
    else {
        wg.RootTable.AllowEdit = "yes";
        wg.RootTable.AllowDelete = "yes";
        wg.RootTable.AllowAddNew = "yes";
    }
}

You are right about disable editing and deleting in the client side but not for enable them. If I invoke JS function in InitializeLayout server side event to enable them, they will be disabled. So I have to remove the invoke script line, and then it is working. Here is the updated code snippet.

How come enable editing/deleting doesn't have to set in the client side but disable does?

Thanks!

protected void _webGrid_InitializeLayout(object sender, ISNet.WebUI.WebGrid.LayoutEventArgs e)
    {
        short fiscalYear = short.Parse(_fiscalYearList.SelectedValue);
        if (fiscalYear == 2009)
        {
            e.Layout.AllowAddNew = AddNew.Yes;
            e.Layout.AllowEdit = Edit.Yes;
            e.Layout.AllowDelete = Delete.Yes;

            //comment out this line, will make it to work.
            //_webGrid.ClientAction.InvokeScript("setWebGridAccess('write')");
        }
        else
        {
            e.Layout.AllowAddNew = AddNew.No;
            e.Layout.AllowEdit = Edit.No;
            e.Layout.AllowDelete = Delete.No;

            _webGrid.ClientAction.InvokeScript("setWebGridAccess('read')");
        }
    }