Roberto we are having the same problem. I think this causes confusion to the administrators since the user(s) appear in drop down boxes (such as in the group administration) although they have been deleted.
At the time I got the request for this todo I looked into the acs-admin but because of future OACS upgrades I hesitated to customize this. However, now we are most going to redo part of the admin section within the next four weeks. What I could do is to forward those files to you Roberto so you can decide whether this is something we want to use for the future.
/Gregor