There is a new video interview with Jason Isaacs online from the Wall-E premiere.  According to Leaky he has

confirmed he will not be seen in “Half-Blood Prince,” he spoke again about his return in the role of Lucius Malfoy for the “Deathly Hallows” movies. He joked while he has not yet seen the scripts, if he is not in the films, he will be “picketing the set for sure.”