That's what you have a Group for. Unless this guy is stronger th*l of your Group, find him in Sage; rob him when he has a red circle from plundering you. Don't entertain him asking you for free wins in things like Matsuri. Continue to take from him until he gets the message that plundering you isn't worth the hassle. Make sure your strongest Group member is on THEN run convoys so they can support. In fact, wait to run your convoy until they've accepted your support request. You can sit in your Group and just wait for that notification they have. Guys like that will continue to take until they begin to lose more than they gain. Worst case you might just want to level freeze for awhile and let him go up. He can only plunder fifteen levels lower than his own(or higher as much as he'd like). If you're out of that range, he'll have to find a new target. Now if he chooses to do much the same in not trying to level, then you know you something isn't on the up and up.