Nationalities

Empires that border mine seem to mix their nationality into my cities even if I don't have an open border agreement and I find it really annoying especially if I want to go to war with them soon. It seems like having a high culture can help you retain your own nationality but you should really have more control over it and there ought to be some type of ethnic cleansing you can do for both nationalities and religions in your cities that you don't want.

Aside from culture does anyone know any ways to prevent this?

There should be a building called Border Patrol and u get to deport a high percentage of the illegals.