Other forms: harassers
A harasser is someone who bothers or intimidates another person. If someone leaves you threatening messages on social media, you should report your harasser.
To harass someone is to bother them beyond mere annoyance, and a harasser intentionally torments or frightens their victim. If your neighbor leaves a rotten egg in your mailbox daily for a month, they're a harasser. And if a bully teases a smaller kid to the point of tears, they're also a harasser. You may be tempted to accuse your parents of being harassers when they ask you three times to take out the trash, but you'd be better off just finishing your chores.