Rather than imposing a new context, you can use the client's own resources to find a new context. Your client says «I want to stop X–ing.» You ask «Is there some place in your life where behavior X is useful and appropriate?» If the client answers «Yes, there are some places, but in other places X is just a disaster," then you know where that behavior belongs. You just contextualize that behavior, and substitute a new pattern of behavior in the contexts where X was a disaster.
If the client says «No, it's not appropriate anywhere," you can assist him in finding appropriate contexts by giving him specific representational system instructions. «See yourself performing that behavior and listen to it. … Now, where did that happen?»
«Oh, it happened in church. I stood up and yelled 'God dammit' and then they came and dragged me out.»
«All right. You know that standing up in the middle of a group of people in church and yelling 'God dammit' didn't work out very well for you, and you don't want that to happen again. Let's find a place where it would be useful for that behavior to happen. You can see and hear yourself doing it in church. Now I want you to change that background—the pews and the altar and the interior of the church—to something else. I want you to keep substituting other backgrounds for that same behavior, until you find one in which if you stood up and said 'God dammit!' every part of you would agree that that is an appropriate response, and you can see, by looking at the faces of the people around you, that others also consider it appropriate. As soon as you find a context like that, then go inside and ask the part of you that makes you stand up and yell 'God dammit' if it would be willing to be your primary resource in just that context.»