I'm a player in a campaign and another friend that is DMing does not have content on DnD Beyond.  I have a Master Tier subscription so I joined his campaign and enabled content sharing.  But, there is another campaign i dont want to share with the party as we're playing that with another group.

I don't have any option to 'block' the content and as much as I trust them, curiosity may get teh best of them and spoil the campaign.

Am I missing something? How can I block shared content as a player?

The DM can block content from the campaign page (should say something like "content manager").

I have the same issue.  I'm DMing a campaign and have master tier access and can share and block/share item by item with my players and this works fine as the DM.  However, I am a player in another campaign (where the DM is also a player in my campaign) and I want to be able to share the sourcebooks, but not the adventures in that campaign, but I can't stop them from having access to the advnetures and content I am DMing, which is stupid.  Even as a player, when I share MY content, surely I should be able to block/share and control what I share with the rest of the campaign - I don't get it?

This reply contains accurate information on who can restrict content access (them DM) and how (from the Content Management menu). Please be respectful when replying to other users, even if you feel their responses are not what you are looking for (alternatively, refrain from replying unless you have new information that may progress the discussion or find a resolution).

There is no way for a Player to restrict what content is visible to members of a campaign, that is always down to the DM. If you have content you don't want the DM to be able to access (say they're a DM in one campaign you're in and a player in another), you will have to either ask them to disable the sources or not look at them.