Actually there's many ways to get rid of a general you don't like...assassinate him, use an inquisitor to burn him for heresy, or drop a spy on him and get him done for treason! But, all of these are quite risky and can be time-consuming (and of course you need to tech up a bit), and are best reserved for 'training' the relevant espionage unit, or when you need to get rid of a very powerful general who might rebel if you just strip him of his title. For crappy peasant governors, the emissary/disband works fine...