This seems to be handled by MainMenuBarArtFrame. Have you tried unregistering its events?
Code:
MainMenuBarArtFrame:RegisterEvent("UNIT_ENTERING_VEHICLE");
MainMenuBarArtFrame:RegisterEvent("UNIT_ENTERED_VEHICLE");
MainMenuBarArtFrame:RegisterEvent("UNIT_EXITING_VEHICLE");
MainMenuBarArtFrame:RegisterEvent("UNIT_EXITED_VEHICLE");