In this world you can never please everyone. There are many ways to politely saying “No”.
You can start by saying “Thanks for the opportunity but I’m afraid I’m fully booked” or “This type of project isn’t my strong suit”. If you find it appropriate recommend someone else for the job. In these type of situations “No” is always better than a reluctant “Yes”.
You could then thank them again for thinking of you, that way you won’t be burning any bridges either.