Server with limited access.
People with low prestige can't participate.
People who have been banned without getting unbanned at at any server are also can't participate.
People with accounts that are being ran for over 80% of play time by sitters are also can't participate.
Basically this ensures that almost no multis will participate.
Finally another (additional) option would be to have a small registration fee. You get the registration fee back in the form of gold. Basically you just have to spend 5€ on gold to participate.
I think this will help against the multis that just serve to feed a main account.
An alternative would be to only have accounts sign up that have bought gold in the past.
In any case. A game round focused on denying multis.