Trim back only to the point that the green growth is still growing. Do this annually for best results. Air and light must be allowed to reach "inside" of spruce in order to remain healthy. Trimming this spruce leaving "pom poms" allows for this to happen. These trees grow slowly.
Pruning an Alberta spruce is just like pruning a tree. However there is a technique of doing it for you to display its beauty and aesthetic value. Do not over sheer the greener part to the point of exposing the brown part.