There are a number of such systems that do this via revoting or dummy ballots. One of my projects, Votexx, uses vote nullification (or flipping) via a trusted t

There are a number of such systems that do this via revoting or dummy ballots. One of my projects, Votexx, uses vote nullification (or flipping) via a trusted third party chosen by the voter. The general idea for all of these is if you add uncertainty you reduce what a coercer is willing to pay creating a mutually assured destruction scenario whereby the system being in place ensures nobody ever tries it. Votexx.org if you want to learn more.