Zergs occur for one reason alone. Being able to jump from server to server has definitely been the cause of this problem. A 50+ man zerg can jump from campaign to campaign easily in a matter of minutes. How to break up this problem?
Remove the ability to jump from server to server even when grouped. Simply put....if you are homed in a campaign, then THAT is your campaign. To jump from one to the other, you must re-home a different campaign at the cost described. Let's face it guys...we really don't need to be in more than 1 server. It's not necessary and causes the zerg groups we see now. I've never understood the campaign jumping of entire groups from one server to another. Not only that, people can with the same account, jump from faction to faction and server to server all night long. Zerging maps a problem for you? Stop the server hoping ability.