Sorry about the late reply, I've been busy and haven't checked the forum for a few days.
Frank is basically right on the money here, but since I was the moderator on the scene I figured I should reply as well.
As Frank says, cities other than your main city are not owned by you, other people are allowed to build there.
And that city with your buses was also one of the biggest cities on the map, so it was entirely reasonable for the other player to want to build an airport there, it was not just picked to troll you.
Also, the other player deleted that airport when you started to make a stink about it, which is not typical troll behaviour. In short, I didn't see the other player do anything that could be considered against the rules.
What you then did was build rails and roads around all "your" cities in order to prevent other companies from building stations there. This is a form of blocking and is against the rules, and I told you as much.
Instead of removing those blocks you then decided to reset your company and leave the game.
Now to be fair, I only joined after you made that admin call, and I don't know if the red player did anything else to rile you up before this. But from what I saw, you were the only one breaking the rules.