https://www.wired.com/2010/08/robot-laws/

Last.fm's robots.txt file includes the following Easter egg:

Disallow: /harming/humans<br /> Disallow: /ignoring/human/orders<br /> Disallow: /harm/to/self

There were discussions about opt-out mechanisms in the Web Annotation Working group (see here), but in the end it was decided that further work would be deferred to to a future version of the specifications.