volt.fm uses two sets of permissions to interact with your Spotify account: Basic and Extended
When you first sign up, you'll be asked for Basic permissions. These permissions only give volt.fm read access to your Spotify account. volt.fm will not be able to modify anything.
When volt.fm needs to make any changes on your behalf, you'll be asked to grant Extended permissions. Some actions that require extended permissions include: following users and playlists, saving artists and songs to your library, and creating playlists.
To downgrade volt.fm's access from Extended to Basic, simply revoke its access and sign in with your Spotify account again.
For more information, please see the Spotify Docs.