Verified answer

There's no way to PREVENT another user from posting ... anything.

All you can do is report content that is in violation. Typically it takes at least 2 users reporting before content is removed, and since comments are rarely noticed or read, it's a longshot for getting a comment removed.

You can prevent specific users from commenting on your own resolved questions. You can do this by blocking them. Simply click their username or point your mouse at their avatar and click "Block user". That will prevent that user from commenting or answering your questions with that account for as long as you keep the block in place, which could be forever, if you so choose. You can only have up to 200 users blocked at any time.

You can't since it's Yahoo property. Just ignore them.