I’ve just had a go on my test site, and this looks like it’s possible. 
I went to /admin/plugins/gamification and edited the leaderboard I wanted to exclude the users from using the pencil icon. In there I left ‘include groups’ blank, and added my custom group to ‘exclude groups’, and that gave me the leaderboard but with just those couple of test users missing.
It’s quite a new plugin, and may not suit every community, but hopefully this helps. 
