One word. Filters. Just seperate the people/communities on your friends list into, well, a Communities group and a Friends group. THen when you chose the security, go to 'custom' and it'll bring up some tick boxes of your different friends groups, and you click the one with the friends in. That way the post won't end up on a Communities Friend Page

Hope that made some kind of sense. 'Cause I don't think it did.