Unfortunately, this is simply not a bannable offense. While we do not encourage such tactics, we cannot enforce banning for it because players are simply smarter than that. If we ban subs, you'll simply run into people making "brother" alliances that simply made an era long NAP with the other team. If we ban that, then people will just be "allied" for convenience. And then things get murkier and murkier as people look for grey area we can't enforce.
Discouraging subs should be something that the community as a whole tries to do. When a world notices a team that's bringing a lot of subs in, they should group together to take that empire down 1st and foremost. However, usually teams don't ban together to do such and instead continue to fight each other until it becomes a 1v1 vs the team with subs (which then really turns into 3v1 or so).
We try to discourage subs where possible though. But at the same time, sub's are great places for newer players to learn while allowing a main alliance to play with a fully serious team.