When I study a new chapter of VOCABULARY, all the new words are automatically saved to "My Words" and afterwards I can delete the words I already know when I go to "My Words". But when I study a new chapter of KANJI (the words I have to write myself), they don't appear in "My Words". How can I delete the Kanjis I already know?

Use the magnifying glass when you come to them while you study.

or bump it self grading with "too easy".. no real reason to delete something unless it is a total waste of time

If you add words from a list, you can also go to the list's words (in the content tab of the list), select the words you want to delete, and press the delete button.

thanks!
I've found the answer to my problem. when I want to add new kanjis, I have to "study" them otherwise they won't appear in "my words".

btw, what is the magnifying glass?

While you're practicing, it's in the prompt toward the upper right, next to the star and speaker buttons. You can click that to get the word popup.