With your nondominant hand lay the drill flat on the edge of a table such that chuck hangs off the edge. Hold the head of the drill tightly pushing it against the table (not the part with the battery).

With your dominant hand, take a rubber mallet and strike the Allen Wrench hard. You want to hit the Allen wrench so that it will move counterclockwise in reference to someone looking at the face of the chuck.

It may take more than one hit but the chuck will spin easily once broken free of the threads of the drill spindle. Use the Allen wrench to spin the chuck counterclockwise until separated from the drill.