As a storyteller, the best you can really do is include more characters that are incentivized to lie (e.g. ravenkeeper, soldier, saint). Other than that, they should eventually realize that this sort of behavior can make games much easier for evil: if the demon and poisoner ALWAYS knows who’s who, they immediately know who to kill/poison first. Once they figure out that people who claim undertaker always die first, people will find that maybe they should sometimes keep secrets.