I hate to burst all of your bubbles, but they are going to have to want to stop lying. You, nor therapists, or anyone else can make them do this. You can give them pep talks, you can scold them, punish them, but it won't work. They have to realize that what they're doing is wrong, not just because you said so. The only thing you can do is be supportive, compassionate, etc for the time being. Then when they finally tell the truth, reward them for it, no matter how good or bad the truth is. It will teach them that telling the truth gets them somewhere faster than a lie.
Also, keep this in mind, children and adults, lie for a reason. In some cases it's because they're not receiving enough attention, etc. Really examine how the child is being treated, if there wasn't a source for the behavior. Well I suppose even if there was an outside source. If it was taught to them from a parent, then you'll really just have to tough it out, and let them learn the lesson on their own. It's very difficult reversing years negative surroundings, but years of positivity and awarding good behavior can do just that.