I don't know. Is there a NAS involved?
By making sure the folder you don't want imported isn't in a folder that is being watched by MC for auto-import.
You can't exclude folders within folders being watched for auto-import. You have to make sure they're not in a watched folder.
Hi AD, I have, amongst others...
Watched: F:\Incoming\Music
Excluded: F:\Incoming\Music\Upgrade
Watched: D:\Pictures\ [System Images Folder]
Excluded: D:\Pictures\Lightroom
And these have both worked perfectly for more than a decade. Why might your experience be different?