Naumov told a news conference: "Amnesties apply to people who receive no disciplinary reprimands while serving their term. Kozulin has had four."
"There is, therefore, no reason to apply for an amnesty. If someone comes up with a proposal for a release with proper grounds or support, we will carry it out."
Kozulin was given three days' compassionate leave to attend the funeral of Irina.
He was on hunger strike while awaiting a decision on whether he could attend the ceremony. On returning to prison he abandoned plans to stage a further hunger strike to secure his final release.
In 2006, he refused food for 53 days to draw attention to human rights violations in Belarus.
Alexander Lukashenko, the president, has called for better relations with the West since quarrelling with Russia last year over energy prices.
Western countries demand in turn improvements in democratic standards, including the release of "political prisoners".
Several opposition activists have been freed recently, but Lukashenko said that the issue for Kozulin had been closed after he and his wife refused the chance to travel to Germany for cancer treatment.
The couple said the departure would amount to going into exile.
Lukashenko, who remains broadly popular, says his stand on dissent and provision of generous benefits have shielded Belarus's 10 million people from the upheavals of other ex-Soviet states.
Kozulin, who ran for president in 2006, was arrested on the last of four days of mass rallies denouncing Lukashenko's re-election after urging protesters to march to a prison where activists were being held.