Here's a good example of a small intervention with a surprisingly big effect, spotted by Jon Strande.

Kevin told me that he's had a problem with flamers, who come to the site and start posting angry comments about other people. They get out of hand very quickly.

Here was his answer:

He pre-filled the comment box with the simple phrase "Everybody needs a hug." Turns out that this small step reduced flame postings by a huge number.