There is no way to implement verifiable voting which is not vulnerable to voter coercion. If you can verify your vote, so can the person threatening you.
The way to ensure the ballot is secure is to have a fully publicly verifiable end-to-end process. You should be able to watch the ballot box from the moment you put your vote in, all the way to the count.
The way to ensure the ballot is secure is to have a fully publicly verifiable end-to-end process. You should be able to watch the ballot box from the moment you put your vote in, all the way to the count.