The Users cannot be deleted as they are either raised issues or being assigned. I would suggest to have a option of disabling the user so that he cannot access the project.
I know there is a work around here by removing the user groups assigned to the user. But it would again take time to reassign the required groups. just by disabling the user i can avoid some time spend on reassigning. This will be further difficult if the user is assigned to too many projects at different levels.
Also i would like you to ask is there any work around for deleting the user permanently having the issues assigned to him earlier. Which should not change.