"1. Go to Story Properties and go to the bottom. Click on the link that says [change editor features] .
2. Change Variables to advanced.
3. Make 3 links on the page you want.
4. Go to the little red stop sign next to one of the links and make sure you're on the page that says Link Restrictions in the pop-up window.
5. Above the button to save and close there should be some text saying Maximum number of uses and you click the drop down arrow and click number 1. Do that to all the links.
6. Now they won't be able to choose it again if they go back to that page."
-- BerkaZerka's article on Link and Variable Restrictions. http://chooseyourstory.com/help/articles/article.aspx?ArticleId=106