Virginia,
This is as simple as a filter. If you specify that the Resource within a work item is inactive and do not want to see that user any longer, you need to expand the Resource Panel and then modify your filter. Click on Manage within the filter settings to add the <Resource>.State field which references the Active/Inactive state on the Task and check Active so that only Active Resources are seen.
Let me know if you need further help.
Sincerely,
Boris