A site-wide official definition of "hijacking" has to be agreed upon. If a rule were just to say:

"Hijacking any thread is disallowed. If any thread is hijacked, it will be locked. Repeat offenders will be punished."

... or words to that effect, well where is the definition of hijacking? Conversation does naturally ebb and flow, and threads adapt to a slightly new topic under the umbrella of the old one when the focus of discussion shifts. How does a moderator decide when a thread topic is being adhered too, and when the thread has been pulled off-topic?

Of course, there will be scenarios where it will be blatantly obvious. Case in point: flame wars. When somebody resorts to a personal attack, and the thread degrades into back-and-forth insults between two posters, then a thread hijacking has taken place. However, in that situation, I've often found it's better just to let everybody get arguments out of their system, and they run out of steam, and the thread dies. Locking the thread means the two posters do not finish their argument, and the pent-up anger will rear its ugly head again in another thread, invariably igniting another flame war.

This is the sort of situation that has to be handled carefully. I'm not so sure creating a rule against it would be the best plan of action.