Simple. "Technobabble technobabble technobabble." Problem solved.
I figure Janeway would fly close to the supernova and keep threatening to self destruct the Voyager until the supernova backed down. Alternatively, she's find some timey-wimey plot device to ensure the supernova blowing up Romulus was just a dream, you know, like the year in hell and a dozen other major catastrophes that never actually happened over the series run.