What if you just don't let them evolve until they've decided they really want to, then?

[There's hitting "B" to cancel an evolution, but--]

I don't have much experience with it yet, since it's new to me, but there's a thing you can get called an Everstone? It keeps Pokemon from evolving when they hold it. You can't do take-backsies on evolution, yes, but there's always prevention.