The Serie A outfit had been linked with a move for the 23-year-old towards the back end of 2013, with the Naples club thought to be keen on tying up a deal in the January transfer window.
Napoli are third in the table and only Juventus have scored more league goals than the 38 registered by Rafael Benitez's men thus far.
The Spaniard is thought to be keen on reinforcing his defence, with N'Koulou tipped as a potential addition.
But the Ligue 1 club are reluctant to allow one of their prized assets depart the Stade Velodrome.
Speaking to RMC, Labrune said: "We had an offer from Napoli in mid-December which was for more than €12millon and was not acted upon.
"There is little chance of him going."
N'Koulou has been an ever-present in the league this term for Marseille and also started in five matches of their unsuccessful bid to progress from the UEFA Champions League group stages.
