I created a bunch of homebrew content for a specific campaign (Harry Potter themed), but somehow the spells have made their way into campaigns that I am just a player in, which is causing some frustration for the spell casters whose lists are now inundated with Harry Potter spells, even with homebrew content disabled in the character builder, and even with classes other than the spells are designated for. Is there a setting somewhere I have to turn off so that other campaigns don’t have access to my homebrew spells? Or is this a bug?

As long as they are in your “collection” then they will be available to everyone in every campaign you are in. To prevent that you have to remove them from your collection, and then add them back when you need them. Any character that already has that content will be unaffected by your removing the content from your collection, provided that it is not removed. If that happens, it will only be available to add back while the Homebrew is in your collection.

That would require removing nearly 200 spells, sub-classes, and backgrounds from my collection. There’s no way that is a feature. I had to click “enable content sharing” in a campaign in order for the PCs who joined it to be able to see my content. Plus the PCs have homebrew content disabled in the character builder and the spells are still showing. This has to be a bug. 

You shouldn’t even need a subscription for your homebrews to get shared within campaigns.

But if the individual character sheets have homebrew content disabled, and the content is still coming through then that is a bug.