Listen carefully and use calming statements like, "I'll bet that was frustrating." and "I can see how you would feel that way."
If the volunteer angers me with a particularly mean critique, I step away for 24 hours before responding.
I try to remember that if only 2 families are complaining, then the other 498 families are happy
Sometimes our principal will offer to meet with the parent. He is very calming and listens and repeats statements like above. Rarily does he need to take action, just makes the parent feel heard.
When explaining what happened or what went wrong, I use the word "volunteer" often. ie. "I'm sorry that there is an error in your directory listing, when the VOLUNTEERS enter the data this sometimes happen. We certainly regret any inconvenience this may cause."
You really need to have a thick skin. Chairman will call me and go "someone complained about this . . . it doesn't bother me but . . . " and spend 20 minutes venting about it. Well obviously it did bother them. Let it go.
Most of the time, a complaint stops with ME. I could call/email every other board member and stir up trouble/anger about the ungrateful parent, etc... but that doesn't help anything. If is a valid complaint, something we can improve upon, I pass it along. If someone is just venting random anger - I let it go.