Hmm, I have an idea, though it's kind of evil. Have three ways to get rid of foreign settlers/workers. First, you can simply kill them, as I suggested above. No rep hit for this, as you're already at war and civilian casualties are "collateral damage". Second, you enslave them as normal - the mother civ likes you less for this (unless you give them back), and starts to seriously hate you if you disband them. Finally, if you are in one of the more evil governments (communism, fascism) AND you have a labor camp or concentration camp in a city (forgot who made this but I know it exists here somewhere), you can move all foreign workers to that city, disband them, and get the full shield cost back from them - this would cause all other civs to like you a lot less, and the mother civ to hate you forever.