I'm not familiar with the details of copyright law. But I would think that Linus has contributions in all part of the kernel. It should be possible for him to claim copyright ownership on all of these part, effectively making it impossible for anyone to legitimately use any of those parts of the kernel against his will. Since that probably (!) also means some essential core parts, I would consider that nobody legitimately uses Linux against Linus's will.

He still cannot claim copyright ownership of the whole though, since he did not create everything in the kernel. Commits from other authors still have their own copyright.