What difficulty are you playing on? On higher difficulties, AI is pretty much immune to WE. Warscore is not really important, and if you are already technically superior, just build a few ships and smash their small blockading fleets one by one. You will not get lasting peace until you are significantly stronger then them, and pretty much the only way to get a good peace out of them is usually invading europe. Once you can keep your costs clear, they are not really doing anything to you anymore even if you are at war, and you can maybe even get a few colonies of them without invading europe once the war takes long enough, but building up a good fleet should always be your goal.