Thank you for the offer I'm sorry I'm afraid it's not possible Thank you for your understanding.

They are many ways to say 'no' politely . The simplest rule is to say ' something nice + but , but im sorry , but too bad..etc later eg That would be nice , but im sorry that i have to.. Thank you so much , but ..... It would be great , but... That's a fantastic idea , but too bad i have to....