If you truly want to warn other Steemians about this blatant ripoff, my recommendation would be to take the emotion out of your postings and let the factual data speak for itself.
1 - you gave him the btc
2 - you got deactivated on slack
3 - you have screenshots
let the proof back you up.
at the end of the day, all you can do is give your story. there's a million guys like Christian out there ripping off unsuspecting newbs. You won't be able to do much to conrol that. All you can do is give a factual account and share your warning. I got hacked the first week I joined steemit, which I still don't know how because I never gave out my passwords.
Thanks for spreading the word.