When users are deleted they are placed in a recycle bin and are removed from any projects. Once users are in the recylce bin, they cannot be created again using the same email data as this would create duplicates. If you want to give access to a previously deleted user again, you have the possibility to restore the user.
If necessary, you can permanently delete a user. Be aware that the action of permanently deleting a user cannot be reversed. When a user has been deleted, all issues which were assigned to them, will be assigned to "Anonymous', and the user will be removed from any approval lists.
To permanently delete a user, go to the recycle bin from the Users page , click on the menu button and select ‘Delete permanently’.