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They can do this because you gave them posting permission when you signed up to use Streemian.

okay i just sign up on steemians account when i was newbie. I have no idea about that

Hehe, you should always be wary of who you give your keys away to. This example was not even that bad; with the Posting Key someone can easily ninja-upvote their own content, or spam posts from your account.

holy shit... U should file a case against them :P

hey that my username ;)