There are basically several things you might want to. Each of these two controls has a code behind file, and a OnDataBound event. In this event you might check if the current item's level is higher than the one you need, and hide the item if necessary. Here are the code behind files for the controls you need:
As for the SiteMapDataSource, you might set the property StartingNodeUrl="~/SiteNav.aspx" in the same code behind files.
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