Q: Do I get to use the Zombie Repellent once each turn, or just when I put it down?

A: Zombie Repellent is not a “use” card. It is a permanent feature of you as a player while you have it. As long as you have it in front of you, it’s not possible for you to have any zombies in front of you as well. They just bounce off of you, and you send them to someone else.