When they got to Water7, they got the Going Merry checked out by some shipwrights and they said they could do nothing to repair the ship, so they had to get a new one. Usopp was against scrapping the Going Merry, so him and Luffy fought for the ship and Usopp lost. So he was kicked out of the crew for going against captain's orders or something around there. After that Robin willingly got taken to Enies Lobby which is like one of the marine bases/prison things. Usopp went along with them, but he wore a mask and called himself Sogeking. Everyone but Luffy/Chopper knew who it was, but they didn't say anything. He helped them fight in Enies Lobby and burned the World Government's flag with his new pachinko weapon on Luffy's orders. After they got away from there, new bounties came out and the one was for Sogeking and not Usopp since he didn't take off the mask the entire time he was there. But he apologized as Usopp and was allowed to return to the crew on the new ship.