Where are people getting all those brahmin from?
This mod lets you disable provisioner, settlement and/or caravan brahmin using a FOMOD installer.

Compatibility:
Edits the Workshop Parent Quest, so anything that also edits it could be incompatible. It's a very simple edit though, so just take a look in xEdit.
The caravan option should be compatible with everything as long as it overrides other mods that edit the same records.

Works on existing saves, but it doesn't despawn existing provisioner or settlement brahmin.
You can just assign the provisioner to something else to despawn the brahmin, then reassign them to the supply line.