How do I block someone from posting on my facebook wall?

[Rich]

To block someone:
From your Account dropdown menu, select Privacy Settings.
Next to Block Lists, click Manage Block Lists.
Enter the name or email address of the person you want to block in the appropriate field and click Block. People will not be notified when you block them.
If you can’t find someone using this method, try going to the person’s profile page. You can block them by scrolling down to the bottom of the left column and clicking the "Report/Block This Person" link. Once you click this link, check the "Block this person" box and then click "Submit."

If you still can’t block someone using the methods described above, it’s possible that they have left Facebook or have restricted their privacy settings. You can still prevent them from viewing your profile by adjusting your privacy settings.