So I made a campaign and bought the Player Handbook to share with my players. What I didn't know is that it is not limited to my content, but all my players' contents. One of my players has all the source books, and despite me blocking them in the Content Management option, the content of his source books are still available for my players to pick and use, even though I have them all blocked. It's quite problematic because I only wanted the Player's Handbook source book to be available to my players. I do not understand why blocking the source books does not work. Am I misunderstanding how Content Management works? Is it a bug? I'd really like some answers.

You are misunderstanding how content management works. When you block a particular source, it only blocks access to read the compendium for that source. There is currently no possible way to block access to player options in the character builder.

What Sposta said. It is intended to prevent players from being able to read adventure (and maybe setting?) books.

People have asked for a better content filter. They know we want it.