Just to be clear, this particular user didn't ever become a fountain of sweetness and light - they were pretty touchy and cranky at the best of times, if I remember right (it's been over a decade), but accepting them as they were let them become a contributor instead of toxic.

Honestly I have thick enough skin that I'm happy to let them be themselves as long as we can reach a basis of professionalism and get a positive result.

You're right that there are many people you can't reach, and trying is a waste of effort, but I think an appreciation for human dignity requires me to at least make the attempt, and sometimes you're rewarded.