A hacker or modder joined my realm

Hello, I am here to report suspicious behavior going on in my server, recently I've had a certain person leave very creepy messages and sometimes even attack me with zombies spawning and witches in precisely very lit up locations. Me and my friends have had run ins with this person but he is very intelligent, I was hoping maybe you would be able to trace this person. And the world is in survival with cheats off so there should in fact be no way that it spawns in mobs. Thank you for your help