Hack threat

One guy was admin on my forum and he was doing some stupid stuff so i took off his admin status. Then he braked into my friends acc and changed forum options and he spammed the forum. We managed to get back admin acc and we changed pw. It is not simple pass now. He registered today again and wrote a post in which he says that if we don't give him admin status he will destroy our forum.

Now i am wondering can he hack our forum without hacking admin acc first?

IP ban him quickly. Ban all his accounts and email addresses. Stop him from coming back. Don't know how he would do this. If the password was simple maybe he just guessed the password then changed a random other account's password so he could get on without you noticing.