What you can do is to make your own version of every official hero into your map and change their team to something like "void" that doesn't exist. If you want, you can copy all of my hero folder in my Enfos and remove the extra custom hero. This will remove all official hero from the picking screen. The downside is that you will need to do it every time a new hero is released.
Otherwise you might need to change the UI so that it doesn't load hero with team Legion or Hellbourne but instead your own. I don't know if you can actually do this or how, since I don't know much about UI modification.
I'll try to look into this when I get home if you can't find it.