Advanced technique for filtering specific communities from the Friends of Friends page (users of accounts with Professional package of service only). [Click here to expand]
Create a new custom friends group specifically for communities you have no interest in watching.
Watch the community or syndicated feed that you don't wish to see. Make sure to remove it from your 'Default View' group, and add it to the throwaway group you created in step 1.
Go back to your Friends of Friends page and see that the community/feed you added in step 2 is now gone. This is because you are now technically friends with it, even though you won't read it on your Friends Page, and so it won't also be displayed on your Friends of Friends page.