The unmanaged detector feature does not clear old entries. The only way to clear them currently is to disable and re-enable the unmanaged detector, or in some cases, delete the client that's used as such.
The problem with this is that this also has the side effect of removing all exceptions that have been added.
When those exceptions are close to 100 MAC addresses, this can be an incredible waste of time.
I would like to suggest a way to export the exceptions to text/csv/excel/etc, and then be able to import them as well. This would make a day's worth of work into a few seconds.