Try this: "I have a policy of not solving my clients' problems for free. I'd very much like to work with you, I handle these issues regularly for other clients. The next step is...." Does it feel easier to do now that it's your firm's policy? You aren't one of those renegades who ignores policy, are you?
Yesterday I was yet again asked advice on how to move someone from prospect to client more effectively. Investigation turned up the fact that the lawyer was offering a lot of advice for free. This is a lose/lose. You don't solve the client's problem and you waste your time. Not only that, but your true clients would be really ticked off to discover you giving services to people that didn't have to pay.
Value your time, pay attention, and don't give your expertise for free. Make it a policy. If that doesn't work, think of your brain power as a tangible product with a price tag attached. How about a bathtub? After all, a bathtub salesperson can't just let you walk out of the showroom carrying one because he likes you or found your bathtub problem interesting!
Make 2011 the year that conversations about money became second-nature.