People have time and talents to share
They volunteer whenever they care
If you must turn them away
Instead of “no” why not say
We’ve love to engage you elsewhere
Customer service training often stresses the value of saying yes to a customer's request even if the answer is no. "I'm not going to be able to do ____, but let me tell you what we can do in this situation." The obvious benefit is that customers feel you're helping them achieve their goals even if you're not completely satisfying their need.
Organizations that rely on the contributions of volunteers to get work done (speaking at conferences, serving on committees, working at events) would be well-served to think similarly. Inevitably, not every potential volunteer can be selected, so plan to manage the rejection so it doesn't lead to dejection. Say no in a way that keeps qualified volunteers interested and engaged in contributing to your efforts.
So if I'm not selected to speak at a conference, point me to other opportunities to share my knowledge. If I am not chosen to serve on a committee or task force, let me know how I can share input or work on projects related to my interests.
Organizations constantly complain they don't have enough volunteers, yet often poorly manage the application and selection of people with their hands in the air. If you want people to freely share their time and talents, carefully redirect their interest and caring when you must say no to their original interest.