If you set this account up with facebook in the first place; then unfortunately you must use facebook; as there is no account to disconnect from. There is an idea to change this on the community though.
If you connected a Spotify account to facebook in the first place; then you can go to Edit>Preferences then disconnect from facebook. This will no longer associate your fb account with your Spotify account.
This post was by Anthony - I do not work for Spotify! If this solved your issue - Mark it as a solution!If you like my post - Please add Kudos! || Spotify Profile - Anthony | Feel free to PM me - Here ||