HoTS 2nd mission, how to stop Nova?
If you have an actual save game from your campaign where you chose Nova over Tosh the game should automatically pick that up. If you don't have a save game you get the canonical options.
This doesn't change her actions though, only her dialog.
There is nothing you can do, either way the action is still the same.
I was curious of that myself and I ran through all the missions on easy on WoL and she said "Although you helped me with my Tosh problem..."