Redmine

Administrators have all permission in a Redmine system, including the ability to change permission for other roles / users. Thus, Admins can delete all objects from Redmine.

If you don't want to grant this ability to a user, you should remove their Administrator flag on their user. Afterwards, their permission are determined by their project roles only which allows you to remove the permission to delete issues. It is generally a good idea to not use an Administrator user account for daily use of a Redmine system.