This is happening to Discord today:

http://gizmodo.com/discord-has-a-child-porn-problem-17936822...

To our knowledge, the only recourse users currently have is to disallow direct messaging

https://blog.discordapp.com/discord-safety-boost-2d592ea3b14...

By default, Discord allows any two people who share a server to direct message each other. Unfortunately, the baddies can use this behavior to send you stuff that you don’t want to see — everything from Harry Potter spoiler memes (Snape killed who!??) to much worse. To keep the baddies at bay, we’re adding Safe Direct Messaging.