How Kind Do You Have to be to a Volunteer?

As many of you know, I do community theater. We are all volunteers. Nobody gets paid even a little bit. Our compensation is the satisfaction of a job well done and a couple tickets for our friends to see our work. So what do you do when people want to volunteer their time, but their skills and/or standards don't meet your quality standards? How do you tell people who have a "this is good enough" attitude, especially when it isn't good enough, that they have to let someone else help? Not the most fun seven hours I've spent. But the show will be ready on Friday.