You cannot stop it. You can only wait.
Quarantine is a good thing if you don't want to spread it further into other cities. Check the unit-card and look for the skull symbol. It will tell you if a certain unit is infected. A city usually takes 4-6 turns to get clean again.
If you want your neighbour to participate in your misery, send him an infected spy. When your spy enters one of his towns, it will become infected, too.

R'as