Server ''hacked''

I played on my friends server today. Out of nowhere weird things started to happen (buildings disappearing, us getting punched etc.) Then we found out it was a hacker. By the way, his username is scatman09. We tried to add him to the banlist, but it didn't work. We have a backup of the world before this happened, but I am afraid that this will happen again. How can we stop the hacker from getting in? Thanks!