Frenchman Niang turned down a switch to Goodison Park in the summer in favour of joining the Rossoneri from Caen on a three-year deal.
His agent, Oscar Damiani, claims the 18-year-old also snubbed a move to Arsenal after spending time on trial in North London.
Niang has barely featured for Milan since joining the Italian club, making just one late appearance from the bench to prompt speculation of a January move.
But Damiani says, while Everton remain keen on the striker and would be interested in taking him on loan, Milan will not allow such a deal - nor will they let him leave permanently.
The agent told Sky Sports: "Everton were interested in taking him on loan after following him already in the summer, but AC Milan won't consider that proposition.
"No offers for a permanent deal have come in, but I think AC Milan would say no.
"Are Arsenal interested too? There was contact in the summer, when he had a trial with the Gunners, but then Niang preferred to choose AC Milan's proposal."
Niang was hit with a one-year ban by the French Football Federation earlier this year following an unauthorised trip to a Paris night club between matches during qualifying for the Under-21s European Championship.
But Milan have big plans for the frontman regardless of that misdemeanour, according to Damiani.
He said: "AC Milan consider Niang unsellable because they believe in his qualities and they want to count on him despite his mistakes."