A new, per-quest option to allow/deny quitting during a quest is available under the "Edit Options" section of the Quests Editor (you may need to update your lang file). If you're upgrading, the plugin will continue to use your "allow-quitting" setting from config.yml as default until you specify a per-quest option.
Huge thanks to
sunta0528 for single-handedly translating over 97% of Quests into Korean, completing the language!