Disable people spying on enemy bases during wars
Nothing irks me more on this game than being in a war and a member from the other clan joins your clan to see what troops are stocked in your clan castle and where all of your traps are set up. Sure, change your clan settings to "invite only" but then that bottlenecks the flow of new members joining your clan as by the time you accept it, they've moved on to another clan.
I feel like this problem creates an unfair disadvantage for the people who play this game the RIGHT way.
My suggestion would be: if clan A and clan B are in a war, a member from clan A can leave an join any other clan EXCEPT clan B until that war is over.