Generally all mail in voting systems let you override your vote. If someone coerces you once and you send in another ballot postmarked after the first or go to your polling location in person on election day you can override that vote.

You basically need to hold someone hostage or under total surveillance from when the ballot is mailed to when the polls are closed to avoid them just sending in their actual ballot afterwards.

With an electronic voting system the window of time you have to hold someone hostage is much shorter - simply force their vote an hour before the polls close and then hold them prisoner for the hour.