Sorry for commenting in such an old thread, but IDK if you've actually thought about this...
What if a player has to go on holidays, so he stays in an allied city, then he would be getting revenue, and thus reducing the amount of revenue for other players... Imagine that when he comes back, he is brainwashed/stops playing/gets bored.
I say that if you're inactive for more than X time (without advice), your character will be deleted/lose reputation/stop getting money/lose money each round and give it to your team. Then, if you say "I'll stop playing for X reason and stay in a city", you won't get revenue, but won't be penalised.
This is in case some people stop playing for any reason and can't or don't want to delete/freeze their characters.
Hope I'm not annoying you.