I tried that and they still appeared in the directory list even though they were inactive users. I've submitted a support case.
For the short term what I did is prefixed the inactive resource last name(s) with "Z-xxx" example "Z-xxx - Smith" so they sort to the end of the lists. This should, I hope, help defer some confusion by making the inactive resources less visible when assigning resources.
A kludgy workaround but it helps...