I've never considered my fork to be a likely theft item. I always assume, perhaps incorrectly, that any bike I see picked apart like that has likely been left in an unsafe area over an extended period of time. I always figure that someone isn't going to go for a full dismantling of a bike unless they can be reasonably sure that the owner isn't coming back soon and that no one else will be paying attention to what they're doing. For that to happen, it seems like the bike would have to be left somewhere overnight and likely for more than one night. Maybe that's not true, but I do know that the only time I found parts poached from a bike of mine was when I left it parked on a college campus for weeks at a time without checking on it. Even then, I think it still had a fork. I've also seen bikes vanish a piece at a time on the campus where I work if they are seemingly abandoned, but, again, I've not noticed fork thefts.
I guess of all the parts someone could strip from my bike, I consider the fork to be a pretty unlikely target, but then maybe that would be different if I had a special fork, but I still have a hard time imagining someone coming along with the tools and the time to make that worthwhile.
But a good way to secure it if you were worried might be pitlock skewers or something similar. Of course if you haven't secured your front wheel, they could just take the fork and wheel together, but perhaps the added bother of separating wheel from the fork would make the theft less appealing. And really that's all you can ever do: make it a little harder/less desirable to steal. There are no theft proof locks.