I do not know of any method to prevent the player from picking up an item that is in inventory (either the player's or a container's). There are methods to prevent the player from equipping the item if that is ultimately what you are after. It is possible to remove/replace the item after the player has placed it in his/her inventory, but that would not look very good. If you give us a clearer idea of what you hope to achieve, we might have a solution that does not rely on the present elements.
I am not certain what you mean by an "animated creature mannequin'" since you specify that you do not want it to move, but I will take a guess. You should set their AI fight to 0 in the construction set (these are unique creature, yes?). Also set the creature's health to something ridiculously high (e.g. 9999). Attach the following script to prevent the creature from moving (head will still move however), responding to attacks and dying:
Begin mor_MannequinScript; Local script attached to (creature ID); Prevents animation, attacks and deathshort OnPCHitMeSkipAnimif ( OnPCHitMe == 1 ) set OnPCHitMe to 0 StopCombat player SetFight 0 SetHealth 9999endifEnd mor_MannequinScript
This script does not provide for the player to activate the creature/mannequin to access its inventory. If the head movement is unacceptable (e.g. these are statues) It might be possible to paralyze them to prevent all movement.