same happened to me, using /undo would kick them with this message from server log:
Object reference not set to an instance of an object. at ConstructionSkin.SpawnGibs (BuildingBlock parent) [0x00000] in <00000000000000000000000000000000>:0 at BuildingBlock.SpawnGibs () [0x00000] in <00000000000000000000000000000000>:0 at Client.OnEntityDestroy (Network.Message msg) [0x00000] in <00000000000000000000000000000000>:0 at Client.OnNetworkMessage (Network.Message packet) [0x00000] in <00000000000000000000000000000000>:0 at Facepunch.Network.Raknet.Client.HandleMessage () [0x00000] in <00000000000000000000000000000000>:0 at Facepunch.Network.Raknet.Client.Cycle () [0x00000] in <00000000000000000000000000000000>:0 at Client.Update () [0x00000] in <00000000000000000000000000000000>:0