You can delete your databases from Strongbox Sync through the App in the standard way by simply removing them. You will be asked to confirm by typing 'delete' because there is no way to recover them after you remove them.
Alternative Method
If you don't have Strongbox installed any more and you want to be sure, you can also remove databases by going to System Settings on your device and navigating here:
Settings > iCloud > Storage > Strongbox > Delete Data from iCloud