Without a lawyer (get a lawyer!) I would remove all references to linked in and add an exception to the extension that prevents it working on linked in (perhaps display a message with a link to the cease and desist message).

I wouldn't bother trying to fight it as a one-person operation. Just accept that life is unfair and carry on. Unless that is you feel very strongly about it and am prepared to sacrifice yourself and future income and potentially employment prospects for the cause.

Good luck.