Why aren't you using the WPF WebBrowser control? It has many Touch events that you could capture and handle and potentially stop the ContextMenu from ever popping up. Or you could supply your own ContextMenu for the browser to use.
The preview events are lower down on the page and can be used to intercept events that would cause the context menu to pop-up.
OnPreviewTouchDown
OnPreviewTouchMove
OnPreviewTouchUp
