I am running into the current problem:

I have a bunch of homebrew for my campaign setting. One of my players is a DM in another game that also has some homebrew for his Campaign setting.

Now his homebrew appears to other player's characters in my campaign, and it's becoming unclear which homebrew they can use.

Is there a way to only allow the DM's homebrew for a character in a campaign. (or only the DM and the player's own stuff).

Unfortunately not, no. All homebrew in anyone’s collection is automatically shared within campaigns. All you can do is:

• Ask the other DM to temporarily remove their homebrews from their own collection and add them back when necessary. (Which can sometimes be a pain in the neck if it ends up happening often.)
• Use something in the homebrews’ names to indicate which campaign they are for. (I use emojis since they don’t affect the URL at all.)

That's too bad, still thank you for the tip!

This is still an issue, can there be made a solution for this?

Still a problem

This is a problem that will not be fixed until they complete an overhaul of everything which for the longest time was "one day" Considering the changes at dndbeyond in the last year, may not happen until they roll out the changes for the next edition of dnd.