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?