"Thou shalt not even listen to generalized criticism from anyone who is not willing to offer a practical suggestion, or their own time, to improve the situation they're complaining about." -RussellB's Prime Directive of Nonprofit Organizing
I mean it.
By taking unconstructive criticism seriously, as a volunteer your morale is impacted to no good end. Furthermore, until pressed to offer their solution, the person complaining will only continue to see themselves as a mere consumer rather than a participant.