I met a provisioner at his "home" town, then assigned him to a guard post to break the line; however, the line is still in my map, and I can't reassign a provisioner to run the route again from either end. Is this something that just doesn't resolve immediately, and might allow me to run another line in time? Any known bugs or workarounds?
I had a similar problem. I tried to cut lines of my provisioners and found it is quite hard to do too.
The console command
player.placeatmewas written in comments. It didn't work with me as the settler appeared but it was not really missed provisioner, but may work for you
If you have DLC Vault-Tec Workshop try installing the Vault-Tec Population Management System and check all the jobs that have been assigned to be sure that nobody designated as provisioner
Try assigned provisioners from the opposing settlement. For example, to make the line from Sanctuary to Red-Rocket rather than Red-Rocket to Sanctuary. You can assign provisioners from either of the settlements - what doesn't work from one direction may work from another
You can install the Fallout 4 mod "Settlement Management Software" and check jobs using that. One of possibilities is to reset the supply lines
Are you sure that the assignment changed? Perhaps you assigned a settler to the guard post who you mistakenly thought was your provisioner. If that were the case then your original provisioner would still be active
Track down the provisioner and transfer them away to any other settlement, breaking the supply line
Kill him? (didn't try). Your settler can't run a supply line if they're dead