Ok, write a status and then just by the side of the 'post' box it shows who will see it. Click on that and then customise (or for less faff, just unfriend them as suggested by bernie).
It'll be easier to unfriend them, as already suggested, but you can post and choose who you want to see it as sherrardk said:
Write your post out and before you click, "post" click on the drop down arrow next to "post".
Click on "custom".
Underneath, "don't share with" you can select people whom you don't want to see your post, by typing their name in "these people or lists".