As Athens I did it successfully by rushing them, but all they have is one Spartan unit, which is not enough for my Hellenic confederation playthrough, so I waited till they had a full stack and then I declared war on them. Killed all their units in battle but I left their Spartan hoplites free to run. They have 4 half strength Spartans and one general unit left and Sparta is under siege but they still won't accept a peace offer to confederate