I suggest that you contact italki support.
Tell them that you made a mistake and posted/wrote something that you would like to delete.
http://support.italki.com
1 กันยายน 2015
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As I know, you can only delete the notebook written by yourself, others(comments, discusses, questions) not.
1 กันยายน 2015
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I don't think you can.. At least I can't find the option anywhere.