Consider that -- just as any particular performer might not suit the needs of your event -- you might not be best suited for the task of saying no. It is highly probable that you know someone who has a gift for that sort of thing, who, in that regard, you might consider soliciting for assistance. One can be honest without being blunt, and if properly presented, constructive criticism can be encouraging. Just as dancers do not only dance with their partners, but with everyone else on the floor (experienced and newcomers alike), the different components of creating an event -- organizers, musicians, callers, sound persons -- can never be in it just for themselves, and should always keep in mind that we all have an obligation to help each other in whatever ways possible.