You can prevent postback while clicking on the root menu by calling client side event on each root menu item which will return false value. Please refer the below code snippet which illustrates this:
$cc1:MenuItem ID="SDate" Look="lookcommon" ClientOnItemSelect="Dummy()" Text="Server-Date"$
//Here we used "$" symbol instead of "<".
Please refer the sample in the below link which illustrates the above:
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