After deliberating for five hours on Tuesday the Court of Cassation confirmed a November order by an appeals court in northwestern Genoa.
The scandal dated 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, had been 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 where he was enrolled for one year.
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.
The anti-immigrant League's coalition partner the Five Star Movement (M5S) was elected on a strong anti-corruption platform.