To expand on John Galt's point, I use a similar tactic, although I tell the client the blunt truth:

Mr. Prospect, now that we have your application on file, I recommend that you allow us some time before you let anyone else to pull your credit. Most of the lenders we work with do not want to see multiple inquiries on your credit report. The commitment I will make to you in exchange is that I will carefully place your file with the appropriate lender, to ensure we get you the best rate and terms. After I secure you a firm approval, you are more than welcome to shop and see what else is out there. Fair enough?

I'm in the library so I'm sure I worded that different than I normally would (I can't say it out loud), but that is the gist of what I reach my reps.