Sometimes Strongbox doesn't completely uninstall when deleted from the MacOS Applications folder (due to an issue with macOS). If you'd like to completely clear all Strongbox preferences/settings then:
Open Preferences and select Advanced > Factory Reset (at the bottom of the window)
Delete Strongbox from your Applications folder
Reinstall Strongbox from the App Store
Terminal Method
For those interested in clearing all the files themseleves, open your favourite Terminal app and enter the following commands:
rm -rf ~/Library/Containers/com.markmcguill.strongbox*
rm -rf ~/Library/Group\\ Containers/group.strongbox.mac*