If you have a site of your own you can use an encryption system to prevent copying, I used 'HTML guardian' years ago and it was very good
As far as posts are concerned I always assumed that once you shared your info on a third party site then you agreed to share with the world, subject to your copyright and a link to your original post being declared
There was an EU case a few years ago, sites copying news stories from the source site (BBC etc) onto their own site were allowed to do so providing they made a link to the original news story available
It's "fair use" stuff, which varies from country to country