I remember someone saying in a beta version you could unsync parts of your library. Is this available now and how do I i use it?
It is available in the current beta of Logos 8.4 which is currently in Release Candidate stage.
I would recommend waiting until the stable release ships.
For clarification: The beta has the ability to download a partial library, rather than the entire library. This isn't the same thing as "partially syncing." All data files are fully synced.