Honor Your Fans
When Star Wars fans wanted to use elements of the Star Wars universe in their fan films, even incorporating story or character elements from unreleased films, LucasArts responded by having a fan film contest in conjunction with one of the biggest fan community websites.
And this was from a company with one of the most valuable brands and stables of intellectual property in the whole world. On the other hand, some companies like to harass fans for being fans. One of these tactics is sustainable in the long term. One is not.