If there's someone Facebook you don't want to have any more interaction with, you can completely block them from seeing you, or you can block them from seeing certain sections of your profile.

To completely block someone, click on the Privacy link in the upper right section of the page. In the Block Users section, search for the user you want to block and once you find them click Block User. Very easy. There is also Limited Profile section which allows you to limit access to specific sections of your Facebook profile.