It is possible to get rid of just the second turn box but you have to mod your Avic, read the faq in this subforum on how to do this. (getting into testmode etc)
The file you need to mod is the data.zip/ui_pioneer/800_480/ui/navigatemap_800_480.ui file, the file where most of the mods in this forum happen. You will find the secondary turn box inside the ui_NavigateMapRouteNavigation layer. You will need to disable 5 lines in total:
- the sprSecCorner sprite, this is the blue background
- two sprites named xxx, the secondary_direction_icon and secondary_exit_icon, these are the little icons at the left.
- two text lines named xxx, the secondary_distance_to_manuver number and unit lines.
If you disable these five lines by placing a ';' in front of them the second turn box will be gone.
If this sounds to complicated, I suggest you just live with the second turn box 