In preparation for building a new Supply Network composed solely of Bots made from the Automatron DLC, I invested significant amounts of time in removing all the human Provisioners.

Most of the Supply Lines I was able to remove without incident.

But two of them remain active and I cannot close them down.

They are:

Castle to Warwick

Taffington Boathouse to Covenant

While I was unable to check the Castle to Warwick route, I did examine the Taffington Boathouse to Covenant route quite carefully.

There is no Provisioner on this path, yet the Supply Line still exists.

It is a known bug that Provisioners can get stuck somewhere in the game, preventing you from erasing the Supply Line because they cannot be reassigned.