Can you steal a number?
Arguably, yes. A safe combination, a gift-card number… Those would, theoretically, get you access to something of value, and thus could be said to have worth of their own, in some senses.
But can a number have intrinsic value?
ISBNs can. $10 a pop, in the United States, in small quantities. More, possibly, in some other countries. (And, conversely, less or none in many others.)
As far as theft goes, it seems improbable, nearly pointless, at first glance. Why’d you want to steal an ISBN? Think about it for a few moments, and I suspect you’ll come up with a few reasons.
All of that ignores the big, obvious question – can you even steal an ISBN, in the first place?