One of my players made a host of home-brew content for a different campaign they're running soon. This is cool, however, all of the spells, feats, and subclasses they made are now visible as options that my players can pick from in my campaign for their characters. I'd like to disable this to reduce confusion among my players, but I haven't seen an option to exclude adding home-brew content made by my other players to my campaign. I haven't seen anyone else posting about this problem but it's been fairly problematic for me, as my players just leveled and a few of them are brand new to D&D. I don't know if this is a bug or simply a feature that isn't implemented, but any support on this matter would be greatly appreciated!

Granular sharing controls s a feature not implemented yet.  Unfortunately I don't expect it to be addressed this year, as it hasn't been on the list of targets/upcoming plans in the Developers Updates.  

If you don't have any homebrew content your players need to access, though, you can just have them turn off homebrew cotent in the first page of the character builder

I have to say, I wish this was a thing. One or more of my players has some homebrew stuff that I wish I could block.

On this note - monsters! 

If I share homebrew content with players (feats, subclasses) I am then also sharing all my homebrew monsters with them, including secret details of homebrew NPCs.
The only way I can see to fix this is to remove items from my collection, which then means I can't use them in the Encounter builder.

Am I missing something obvious?

You can add the homebrew monster to the encounter and then remove it from your collection. It'll still be visible in the encounter builder and combat tracker.

Oh!  This is a neat workaround.  Thank-you. :)

That same trick also works for putting secret Spells and Magic Items on NPCs (as long as the character is set to “private”).

By NPCs, do you mean characters, or monsters?