If an external user has made several attempts to add assets or their Upload Set is no longer needed, you can permanently remove it from the account by following these steps.
Delete an External Upload Set
Go to Assets > External Uploads
Locate the desired set by scrolling or searching
Click the trashcan icon at the far right of the set’s name
Confirm your choice
You can also delete an External Upload Set from inside by clicking on its name and selecting "Delete Set" from the dropdown menu.
This action cannot be undone, so make sure you have the correct External Upload Set before committing to your choice.