After deliberating for five hours on Tuesday, the Italian Court of Cassation confirmed the November order by an appeals court in Genoa.
The scandal dates back to 2008-10 when the party, then known as the Northern League, was found to have fraudulently claimed 49 million euros ($58 million) in electoral expenses.
Northern League founder Umberto Bossi's former chauffeur and treasurer, Francesco Belsito, was charged with falsifying the party's accounts.
The court of cassation acquitted both men because the statute of limitations had run out.
According to wiretapped telephone conversations and testimony cited by Italian media in early 2012, Bossi, as well as his sons Renzo and Riccardo and other relatives, spent the funds on home improvement, renting luxury cars and even dental work.
Renzo Bossi was also suspected of buying a three-year university degree in Albania.
Former Northern League leader Umberto Bossi (L) and his son Renzo in 2010. Photo: AFP
Of the 49 million euros seized by the courts, only three million appeared on the League's books.
The party agreed to pay back 46 million euros, in instalments over several decades: 100,000 euros every two months, or 600,000 euros a year.
Bossi, once a key ally of former prime minister Silvio Berlusconi, was forced out of his party in 2012 after the fraud allegations surfaced.
The party tried to put the scandal behind it by changing its name and installing Matteo Salvini as party leader in 2013.
The anti-immigrant League now rules Italy as part of a coalition government with the Five Star Movement (M5S).
M5S was elected on a strong anti-corruption platform.
Salvini, famous for his love of social media, was widely ridiculed in Italy last month after blocking phrases including “49 million” from comments on his Facebook page.