You can banish a player from your alliance's lands by assigning a task to attack and banish. Tap on the player's castle, then on the tasks icon located at the bottom-left corner of the pop-up and choose Banish Player. After the player is defeated, they will be automatically teleported from the alliance territory (banished). Note that you have to be at least a Task Master in the Alliance in order to be able to set such a task.