Having rejected unintentional plagiarism, I think it should rather be called plagiarism due to ignorance. Maybe, you are unaware of the conventions of citation. Then, you are unwilling to find the appropriate way to cite(plagiarism due to laziness). These are understandable to me, but unintentional plagiarism I do not understand.
Thanks all!
Its case to case basis actually. There are some in the past who committed some plagiarism but since they contributed a lot in forum, Im pretty sure admins have considered it.
Plagiarism is a big sin so those who got banned are probably did with intent their did. If someone did plagiarism, then he is aware of it especially if he didnt understand those terms and condition you stated.