Have them make any ToB character they want (avoid warforged) up to level 6ish, which is the tipping point for damage it would seem. Proceed to kill them in one round by telekinetic thrusting 250 pounds of poison at them as a level 5 psion. Or have a wizard kill them by doing whatever broken stuff a level 6 wizard can do. Just kill their ToB character with an ubercharger. Show them that there's tons of other stronger stuff.