I've purchased some books and some individual items from other books, say only the classes and races from GGtR. This book doesn't show up in my content management for games in which I enable shared content - although the players are able to access GGtR content as expected. 

This is fine for campaigns where I want to have everything, but I'm running Strahd and want to restrict this book from another campaign. Is there a way to disable GGtR content in one campaign? I can disable the books where I've bought the entire thing, just can't find a way to disable a book where I've only bought a subset of its content. 

You can only restrict access to the compendium content of books using the Content Sharing controls for a campaign. It doesn't affect the character builder and search tools on DDB.

However, one sort-of option you have is to disallow Magic the Gathering Content on player character sheets, since GGtR is MtG content. That's about it, though, and you'd need to make sure your players know not to toggle it on.