It's a problem constantly causing my server to crash.

Ok, so, there is this kid on my server named skullridrrr, every time he gets on the server, it causes it to crash. All I know is that it is because he was testing the command to make a pyramid and accidentaly made it out of torches which made the server lag REALLY bad... about an hou later after the server stopped lagging it finally showed up that he had left the game, but now, every time he tries to get on the server, it show about 90000ms lag, then crashes, every time. PLEASE fix this, it's really annoying...