Blocking websites on OS X I would like to block some websites on the guest account, so my cousin cannot browse things that are not safe for my computer.
You can do this using Parental Controls on the Guest account. See Apple's KB article regarding this for the procedure:


OS X Mountain Lion: Filter inappropriate Internet content



  Manage a user with parental controls
  
  
  Choose Apple menu > System Preferences, and then click Parental Controls.
  Click the lock if it’s not unlocked, and enter an administrator name and password.
  Select a user in the list of users, and then click Web.
  In the Website Restrictions section, select the Internet restrictions you want for Safari.
  
  
  To allow Safari to access Internet content without filtering, select “Allow unrestricted access to websites.”
  To filter inappropriate Internet content or websites, select “Try to limit access to adult websites automatically.”
  
  To allow or restrict specific sites, click Customize.
  To limit access to only specific websites, select “Allow access to only these websites.”
  
  To add a website to the list, click Add (+) below the list, choose Add Bookmark from the pop-up menu, enter a bookmark name and webpage address, and then click OK.
  
  To add a bookmark folder, click Add (+) below the list, choose Add Folder, and then type a name for the folder.
  
  To remove a bookmark or bookmark folder from the list, select it and click Delete (-) below the list.
  
  


But to fully protect the computer from potential harm from Internet regardless of account used one has to block any downloads and installations without explicit approval. (I do that to protect my self from my self :)

Each browser has they own set of tools that can be used to protect user and computer. Loots of good add-on's in Firefox. 

As an example for Firefox you would use a add-on ..Block downloads, 

lock down bookmarks/addons/downloads with a password.