By telling them that you are not going to vote GOP in november even if that means that a democrat gets elected.
Elected officials don't care about emails, voicemails or marches. The only thing about marches they care about is if they start swaying public opinion.
The only true thing elected officials care about is how well they poll in a race against the OTHER party. They don't care if their supporters bitch and moan as long as they show up to vote in the general election to keep the other party out of power.
Letters to the RNC might help too if you've been a donor in the past.