You can't control what content players have access to in the character builder, just what compendium content they can view, using the content management option in your campaign.

I would suggest simply telling your players what the guidelines for character creation are. If they choose something that is not approved for your campaign have them change it. 

I’m getting ready to start a new campaign, and I am having this same issue. I can tell my players what isn’t available until I’m blue in the face, if it’s in the builder they pick it. Content management really should cascade to the character builder. 

You have to keep in mind that some of the content in the character builder comes from adventures. If you're trying to keep your players from reading the adventure they're playing by turning it off, they wouldn't have access to the content that they are actively playing, which would be problematic. Ideally, the thing to have are two separate options to activate/deactivate, one for the compendium and one for the character builder content.

I am running into a similar issue with Homebrew content.

I have homebrewed a lot of things to re-skin them for my own campaign setting, as well as making completely unique items and abilities that I don't want my party to see.  The only way to limit their access right now is to have them deselect "Homebrew Content" under the Character Preferences tab of the Character Builder, but since they can go back in an enable that content themselves, I have no way of making sure they don't accidentally or intentionally see that content.  

I would love to have a way to either lock those selections in the Character Builder, or even have an option in the homebrew editor itself to toggle specific campaigns that can see that item/feat/spell/etc.

If they are things you made, you can remove them from your collection. They will still be listed under “my creations” but won’t be shared with the campaign. I agree, though that more control over what is shared would be ideal. 

Just want to add my own support for this as a request.  I think it should be done with a notation in the Visible Notes or somewhere, so the player's aren't mislead by what D&D Beyond offers in it's totality and understand there's an in-campaign restriction.

Quote from RadioKuma >>

I don't run any of the published adventures, so that isn't an issue, but I can see that being a related problem. If you're running Curse of Strahd, for example, it'd be a shame to have players take a peek at Strahd's stat line, for example, or perhaps browse through and see some of the items you can pick up in that adventure.

 You shouldn't have any issue with disabling individual adventures or sourcebooks within the compendium of shared info, and the character builder shouldn't have any option for viewing something like Strahd's stat line.  The main issue could potentially come from them looking up "Strahd" or such and finding an item with his name in the description.  

I really wish there were settings to deactivate content from one's collection.

I never plan to use Acquisitions Incorporated, Rick and Morty, or the Italian PHB and I'm tired of seeing them in my stuff. 

This is an issue that should be fixed by developers, in a campaign you should be able to filter which sourcebooks are available.  A Wildmount campaign shouldn't put Theros content on the PCs list, or vice versa  

Quote from Falcon_Neil >>

This is an issue that should be fixed by developers, in a campaign you should be able to filter which sourcebooks are available.  A Wildmount campaign shouldn't put Theros content on the PCs list, or vice versa  

 Well, that particular example is already possible: Players just need to be told to turn on Critical Role content and turn off Magic the Gathering material.  That said, I would love to see much more granular control of content.  (Like, for example, turning off Aquisitions Encorporated, or ever turning on or off specific character optionsl)