Shorter version:

    news.ycombinator.com##.default:has(.hnuser[href$="=USERNAME"])

Alternative version that just makes the text invisible so that you can still highlight it to read it if you really want to:

    news.ycombinator.com##.default:has(.hnuser[href$="=USERNAME"]) .comment > .commtext:style(color: #f6f6ef !important)