A hacker/griefer is attacking my server

I used to have this friend who owns a server. But then as soon as I created a server he got jealous at me(thinks that my server is robbing him of players). I didn't do anything wrong. Well now he wants to grief my server. He tried to use a hacking client(specifies a username) and logged in as an op but i caught him and ip banned him. I run my server offline mode. I installed login plugin AuthMe. I am planning to get OPPassword(so you need a password to op someone). He also told me that his ip changes every few days. So at this point how should i stop him.