Rather than deleting users, why not just disable and protect them? This prevents them from logging in or changing any of their settings. It has the advantage of leaving them in the system for display purposes. I believe that it uses a strigkethrough font when displaying.

This is what I'm doing at moment, but what is than deleting user good for? I think it doesn't make sense in any project if the real names will disappear and so I can never delete my users leaving company. For our institute we have many students working some months here and than leaving again, so I would like to remove them.

For me the difference would be:
disable user: user is temporily not on the project or in company, but he will most likely come back.
deleting user: user won't most likely come back, but user info should also be kept. Maybe we have a question a half year later to him (which is very common here) and with him as handler we know that he handled the bug :)