OnHeaderMenuShowing

This event is fired before the header context menu is rendered.

Fired by	RadGrid
Arguments	gridColumn - current GridColumn object domEvent - the original browser event menu - header menu instance
Can be canceled	Yes

Example:

<telerik:RadGrid RenderMode="Lightweight" ID="RadGrid1" runat="server">
    <ClientSettings>
        <ClientEvents OnHeaderMenuShowing ="HeaderMenuShowing" />
    </ClientSettings>
</telerik:RadGrid>

function HeaderMenuShowing(sender, eventArgs) {
    if (eventArgs.get_gridColumn().get_uniqueName() == "ContactName") {
        //hide the Clear sorting option from the header context menu of the column with name ContactName
        eventArgs.get_menu().get_items().getItem(2).get_element().style.display = "none";
    }
}