I don't use Facebook really myself, but I would take a look at your privacy settings. You may not be able to block friend requests, but you should be able to make sure that your posts only go to your friends and that your profile doesn't show up in search results.
I use it to keep in touch with certain people and a group I belong to (that I really do not even look at). Other then that not at all. I also do not share any of my info unless you are a friend. So no one can see my activity, friends, pictures etc. Only info that can be seen is my Public postings ie, cover picture or avatar.