Is there any way to ignore user on HN?

There's a contact link in the footer to send an email to hn@ycombinator.com

In my experience, 'dang reads those emails personally and responds promptly.

You could write something in tampermonker/ greasemonkey/whatever to remove the posts yourself.

Flag them and move on.

I can't flag them, there is no such button.

The "flag" link doesn't appear unless you open the comment on a separate page by clicking on the "nn hours ago" link. (It's possible however that you can't flag a reply to your own comment, since you can't downvote replies to your own comments.)