Probably the most "polite" thing you could do would be to suggest the name of another firm that does provide these services. The words "I'm sorry" (or "I'm really sorry" as Ruthi suggests) are enough. Nothing you can say will change reality, it's unpleasant to get a "no" no matter how it's said. In U.S. English we don't use long apologies; politeness is just a matter of using short phrases like "I'm sorry," and longer apologies don't sound any more polite. Tone of voice is important, but hard to teach or learn. If you know a native speaker, you might try saying "I'm sorry, we don't provide those services," using exactly the tone of voice you'd naturally use when speaking Chinese. You don't have to get exactly the right tone of voice. Just make sure that the natural Chinese intonation doesn't sound curt or angry in English.

I would say "I am afraid we don't provide that service" or "I am really sorry but we are not able to provide that service".