3-minute uBlock script:
news.ycombinator.com##:xpath(//span[contains(@class, 'comhead')]/a[contains(@href, 'user?id=USERNAME')]/ancestor::td[contains(@class, 'default')])
Replace USERNAME with the user you don't want to see.
Edit: it only works on comments, but you can just as easily make one that hides their submissions as well.