Your comments

IP ban would be good, but I play with friends on the same network. Maybe a max of three or four?

Also fun fact: Those tribes are not hackers, nor are they controlled by a player. They use an exploit based on a valid feature of the game.

When you refresh the page or close window, you obviously lose your tribe. However, it's not dead. Instead, a basic AI takes over, which will normally try and attack the closest enemy tribe. What these people do is crete a tribe and refresh the page, again and again. If they have a few computers, they can easily pump out hundreds of tribes in a short space of time.