It is currently impossible to delete a user if this user reported or is currently assigned to any issue. Issues can be reassigned by hand (tedious) but it's impossible to change the reporter of issues (see
It would be much easier if the administrator was given an option to substitute the deleted user for another, selectable user.
Otherwise it is really hard to maintain the list of users to a manageable size.
(ideally, two substitutes
one for assigned issues, one for reported issues can be specified)