I am not sure if this is by design or not but I don't have the option to ransack an Independant Peoples city. It is a bit game breaking/wrecking because i was busy colonizing the new world and three aggressive IPs have sprouted up next to me and I can't capture them because of city limit (and they'd be shit cities) and i can't get rid of them either. ??

Just capture them and then ransack while you own them. This is what I was doing.