As for the Server, there is not. The only thing you CAN do, is disable monsters which is the equivalent of Setting your Server to peaceful. Im not quite sure if there is a Plugin for Difficulty, at least I cannot recall something like that.
Thanks, will look into plugins then. Some plugins (such as MineQuest) changes the difficulty quite a bit, even if its not per minecraft standards.
My personal preference is to use the slowhealth plugin along side enabled monsters. The best of both worlds.