There's absolutely no way to stop people from taking your work against your wishes if they really want to...years of Warez/Torrents/File Sharing/etc. has proven this.
But the fact that there are guidelines for sharing content in the official forums (and I'm assuming that since these are TFPs rules, they extend to the Steam forums for 7Days as well) makes a huge difference as that's where people generally go for content.
Thanks for these.