The 23-year-old, who is regarded as one of the brightest talents in Serie A, has scored nine goals in 18 league matches for Fiorentina this season.
His form has not gone unnoticed, with Juventus said to be preparing a bid, but Cognigni has denied claims that the Florence club will sell.
"It's not possible that there will be any negotiations with Juventus for Stevan," Cognigni told La Gazzetta dello Sport.
"In fact, it's not possible to deal with them or anyone else - Jovetic is not for sale. He is happy at Fiorentina and has a contract with us for another three years."
A number of Premier League clubs have also been linked with the Montenegro international.