Hide deactivated users

Recently, deactivated users can now be assigned to things. Why this would be helpful escapes me. Why this would be harmful is obvious.

Let's say I accidentally assign a really important task to a deactivated user, guaranteeing it will be ignored… and late. A great way to anger a client.

I cannot think of a single good reason to assign anything to someone who does not work here anymore and will absolutely, 100% not fulfill the request.