Ruby went through a stage where she wanted to nip us, and every now and then will get in one of her moods where she'll try and do it again. First i try a firm "No!" not yelling or anything, just a firm no, like you would say to a child or something, to get her attention. then if she keeps on, i will indeed place her in the cage, tell her she's in time out, turn the light off and walk out of the room. then about 10 mins later i come back, get her out, act like nothing happened, and she's usually fine and behaves herself. she definitely now knows what "no" means, and will tell us "no" if we're not letting her get her way sometimes, haha. this method of discipline was recommended by our vet.