I have been having this problem with clan hoppers who join for a few days, wait for war to start with them in it, and then hop to a new clan. After careful consideration, and discussions with fellow clan Chiefs, I have come up with the idea of "The Scarlet H." The basic idea is that a hopper is punished and shamed for a behavior that affects clan morale. If a player hops during war and does not return for their two attacks, then a large, red H appears with their name. There is no other punishment then this, just a letter that automatically appears and never leaves. This way, clan leaders know when they are putting hoppers in war. This way, clan recruiters know when a hopper is coming into their clan. This way everyone knows a hopper, because they can be seen.
I want to stress that no other element of game play is disrupted, no resources are lost or given, and the only thing that happens is that hoppers get labeled. Plus, they are only labeled by the game system if they (a) leave a clan, and (b) don't return to that clan by the end of war.
They should have to live with their shame for all others to see for the remainder of their clan days. Clash on, brothers and sisters!






