Correct, you can't delete your account much like you can't remove an address from the Bitcoin blockchain.
Sorry if the plagiarism fighters were harsh on you. It was probably because there was a chance someone could have interpreted one of your video posts as claiming the video was yours.
So if I share any article on here I have to specify I didn't write it? Thats moronic. If that was the case sharing on the internet would be massively tedious as you would have to include this with every link you post.
It looks like you've been doing it right, though when it comes to Steemit related content it could be assumed that you created the video, since it was about Steemit.