That being said if I were in their shoes I'd shut down immediately, put up some angry screed on the project's website, and disappear off the Internet. If someone from Riot's actual legal team wants to demand destruction of source code or whatever, let them do the extra PI gruntwork.
Of course I'm also the kind of person who's massively shied away from fan projects precisely because there are people who not only do not appreciate them, but are entirely willing to legally prosecute you far beyond just shutting down the project and clearing the market. Why? I dunno. There's a weird school of copyright morality that wants to equivocate any kind of infringement with forcible rape, especially with regards to fan projects. It's like some sort of weird dual to toxic fandom.
(It does not help that certain fan scenes, notably Melee, are full of actual sex pests. I still consider it a false equivocation.)