We don't recommend that you pull the Smart Figure off the Power Activator, as it is designed to pop off.
When you defeat a villain, they should pop off the Power Activator automatically. To remove a hero, switch to Exploration Mode and spar with them until they are defeated. They should pop off the Power Activator at that point.
Some missions require that a specific enemy spawns from the Power Activator. If you are using only one Power Activator and it has a Smart Figure on it, the Smart Figure may pop off when that enemy is spawned.