as others have stated a show of strength mite be enough to coerce them into peace, take a city, destroy a full stack somewhere if they have 1 loitering around. or just offer them a heap of crappy out of date techs u dont care about. usually need to offer 3-4 in my experience but who cares about carbines and drumming when u have platoonfire etc :)

if they still wont accept your only choice would be to take a city and offer it back or just offer a straight peace deal after beating them up abit :)

the AI will not accept peace if its not losing :) its a bit like EU we need a warmeter :D so we can see how the war is progressing :D