Sign in to your Facebook account and click on the "Account" link in the top right corner of your screen. Choose "Privacy Settings" from the "Account" menu.
Click on the blue "Edit Settings" link in the "How You Connect" section of the "Privacy Settings" menu.
Click on the gray rectangle next to "Who can post on your wall?" that says "Friends."
Click on the "Only Me" link that appears beneath "Friends."
Click on the blue "Done" button to set your new privacy level. In the future, no one else will be permitted to write on your profile wall.