domination victory

I know a lot of you disagree with this, but me and my friends only like the domination victory part of the game. The problem is, whenever I disable all victories except for Domination, the AI can still get victories in other fields. Does anyone know why that's possible?

Hey I am playing (england) on a huge island map where I have captured all the capitals except one that another civ (america) captured a while back. I can't see that capital on the map and I don't know how to negotiate with America to give it up. Do I have to wipe out America? Spain? America...