SQLSister,
Rewarding them by giving them added responsibilites ... will only increase it not fix it.
Well, you probably have a better insight since you do manage people and I usually don't, but I used to work with a person like that. She thought she was too good for the job, was hiding her knowledge on that basis (she was good at what she was doing, I guess, but beleive me, that hidden knowledge was easily found in help files and professional magazines), complaining to coworkers one day, spreading gossips or arguing bitterly about small stuff another, and the third day she for no apparent reason stopped speaking to the same people she complained and gossiped. Granted, she was usually in a bad mood. Well, I never confronted her, I just avoided to argue or listen to complaints and gossips, and just ignored her not speaking to me. I don't think managers were aware of what's going on. But I noticed that when she had a good assignment and was all busy doing it, it was good for everybody around her. She was sitting all the time in her cubicle, working hard, not complaining, not gossiping - it was just great! (Or maybe she was just not speaking to all the people at once?) So, I guess, some people doing it because they are bored with not enough work.
Stella