The story is kind of weird, but I guess I'll go through it all.
First, his server was down. This was one of those servers where everyone is op (it's all friends). I bugged him about the server being down IRL, so he jailed me when the server came back up. I was curious how jailing would work with op, so I was trying out all of the operator commands. Lo and behold, as I progressed through the list, I eventually did /boom and it worked, and I unintentionally blew up the jail that I was in. I attempted to replace the blocks, but because I was still "in jail", I couldn't build to repair it. After some talk, he unbanned me, only to reban me when I was asking if I could get op again, explaining that there was basically nothing you could do on the server without op (for example, you couldn't even use pressure plates in spawn).
Thus, it's kind of an accidental greif that got me banned initially, I don't even know why he banned me the second time (I think he was still irritated that I had accidentally blown up the jail), but in the end it's not about greifing, and I don't think that I will even be able to do something like that here.
