I'm already over the line and out — is it still fair to sit on a rival up the beat?
No. Once you know you're disqualified — here, over the line at a black-flag start (Rule 30.4) — staying out to deliberately hinder a rival isn't tactics. It breaks Rule 2, Fair Sailing, and can become misconduct under Rule 69. In World Sailing Case 65 the helm was excluded and the boat thrown out of the whole series.
<iframe width="560" height="315" src="https://www.youtube.com/embed/mmm00QUpaQU" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/v/mmm00QUpaQU/">Source: Sailing Kiwi</a></p>