None of those suggestions is neat, nor easy, if:

There is no good enemy assassin waiting at your beck-and-call.
You don't own a city with the plague.
You don't have boats because you don't have a province by the shore.

And I can't keep them as brothers or uncles because they still suck down my limited denarii.

The point of the game is not supposed to be finding frustratingly inventive ways to eliminate your own people. I hope the next Total War game (hint, hint) allows for dismissing or disowning family members.

As an aside, can you dismiss agents?