You can delete a user metadata one by one or a metadata of all users. It is a powerful tool, so use it responsibly. The deleted data can not be recovered, so we strongly recommend making a backup of your database before deleting the metadata.
If you leave the User ID field blank, the deletion of the metadata will apply to ALL users. But if you add a user ID, only the chosen metadata of that user will be deleted.
Now you can select user by dropdown!
This plugin is available for download for your site.