Question: I'm hosting a Minecraft server with 14 friends of mine playing on it. We peacefully build houses and structures, hold events, go kill the enderdragon, stuff like that. All is peaceful. Until one day, another friend of mine (let's call him "A") joined the server. Then our houses got griefed and burned. Our stuff is all gone. We (me, another owner, and 2 mods) took a month to investigate and finally found that "A" was griefing. We talked to him and told him not to grief again or else we'll kick him. And eventually we kicked him.

And now the problem begins. A moderator wrote to me saying that I should give "A" another chance, and that he apologized. So I thought, why not? Then, after I added him back -- on the very same day -- my best friend's house got filled with water. Checking the records, I found out that it was "A" again. I wanted to kick him, but it would hurt our friendship. I mean, we meet each other at school every day. He would be so mad at me. But if I don't kick him, 3 other players will leave the server.

What should I do to solve this mess?

Answer: 
I think it is rather simple. You like this game, you enjoy playing it with other people.
Tell him that, if he values you as a friend, he will stop his behavior, if not just kick him.

People sometimes misses the real meaning of what is going on because they think is only an "online" event. But what if you were building real stuff, and he came and destroy it, would you accept that? What if you were writing a story on your computer, that is "virtual", would you like if he came and deleted it all?

You invest time in that game, is something you enjoy, the game is online, it may not be important, but your time and feelings are, if he can not accept/understand/respect that, do you think he will be able to understand in other contexts?

Value your time and feelings, and have friends that value them also.
Create a second server where you can play and have fun and leave him a copy where he can destroy if that is his prefered way of playing.