Its very hard to prove someone is an alt. I know I reported a bunch of likely alts when a group of very similarly named castles all came out of inactivity at the same time to fireball members of my league (They all had the pink returning player shield on, which is how I know they came out of inactivity). That makes it look to me as though someone resurrected a bunch of inactive alts to take advantage of the lower cost of sapphires so they could fireball players without having to spend as much $.
Alt use/abuse seems very common in this game. People plant alts in leagues to act as spies. People use alts to attack and bully other players. People create tons of alts and send the defense to their main castles so that they have millions of "free" defense (and out of league def too, so they don't have to worry about points giveaway during wars). Heck, I've even gotten letters from players who brag that their castle is defended by alts and that I'm not hurting them at all. It seems that even letters from the player, bragging that they are abusing alts for their own benefit, are not sufficient proof.
I can understand to some extent. Banning a legitimate player could probably cause Plarium massive problems. Just looking at how the game and TS3 are set up, its very easy to change your name or have a buddy (or alt if you're one of the many that use/abuse alts) change their name to create fake "evidence". Not even ISP tracking can prove an alt beyond a shadow of a doubt. Is that an alt, a family member, a roommate, or a coworker?
It would be nice if a mod or community manager would post what info would be most useful when submitting a report about alt abuse. Knowing what information we need to gather to help Plarium catch these cheaters would help us in reporting more effectively.