I've seen the issues of holding, double teaming, and unfair/cheatful alliances brought up very often in the forums as a whole. I believe I have a way to fix this, or at least create awareness about being a good sport.
If users could rate one another, at the end of a game I believe that people would be less inclined to cheat. Sites like eBay have user ratings and if I bought something on eBay I would be much more wary buying from someone rated low when compared to someone with a high rating.
On CC, the players could be rated on a scale of 1-5, based on whether they hold throughout the game, cheat, or whatever. Obviously, much of this would be opinion, but the rating could be hidden for ten games, or untill a good amount of people have rated that specific user.
The way for the holder or cheater to fight this would be simple. Announce in game chat if you know, or think, that you'll be gone for an extended amount of time, and announce alliances. After the alliance is announced, PM's are fine, imho.
Anyway, I truly believe that this could help. People will avoid people who are unfair, and in return, players will try to be better sports and follow the unwritten rules (and the written rules) of Conquer Club.