I have found that loading that unwanted general or king onto a ship does work this way.

Load said unit onto ship and move out to sea. Use the army scroll list to find said general. Have him walk off into the water....

I did this by accident the first time. I have never had them with a stack of units or any other spies or priests etc., just the lone unit I am trying to get rid of. It annoyed me the first time as I lost a good general. After that, I use to get rid of the faction heir/ruler that is a real loser.