If you mean to remove an address that starts to show up automatically when you go to type an address in the "To" block of an e-mail (in the Mail app), then I'm afraid that's a "feature" of Apple's that no one can remove.
We don't know why Apple insists on not providing a way to empty that caches, but they haven't, so here we are ... stuck with it. If it helps, you are among a very large group of people annoyed by that feature!
If that's not what you meant, then never mind and could you please explain further what you DID mean? TIA.