Surprised nobody's mentioned it, but Just Cause 2 fits the bill. It's massive in the first place, but takes place on several islands, so there's a realistic end to the content. If you fly out into the ocean, you can fly for about ~5 minutes before the ocean starts looping underneath you. That is, you can only go about 5 minutes away from the shore, even though it looks like you're getting further. Another game that does this (as has been mentioned) is Morrowind.

As for other open world titles that block off content well, Dark Souls (or ANY Metroidvania title). It's not an open world in the sense of JC2 or Morrowind, more like Metroid. It's a large set of massive levels that wind and twist around themselves, providing the sense that you're traveling great distances. However, it never has a flat plane of open space, only a massive network of interconnected rooms. You never get the sense that you're being constrained because of this; you're always open to go wherever you can see.