Both online and offline, there’s always someone who annoys, bullies or even threatens other people. If this persists through a certain amount of time and is done online (via social media, messengers, chats etc.), it’s called cyber bullying. Most times, this is directed at minors. If you encounter this type of behavior, be it as a target of it or as a witness, family member or teacher: get help.
The following portals are a selection of sources for this specific and other problems. All of these services are free of charge, of course. Please note that these services are in German, while the contacts provided may be able to answer in English.