In real-life history, what stopped people being illegally enslaved in the same way that Tyrion is by Jorah Mormont; just shanghaied while vulnerable and without witnesses?

Not much, to be honest. 

EDIT: To be a bit clearer – slavery is rarely designed to prevent people from being enslaved, which makes the term “illegally enslaved” somewhat of an oxymoron. 

Generally speaking, the only protection was community – were there people around you keeping an eye out who would make trouble if someone tried to drag someone off – and social status (i.e, was there a chance that the slavers would get in trouble for trying to kidnap you?).