How can I block specific telephone numbers on my iPhone 4?
There are applications that do this if you have jailbroken your iPhone.  Without jailbreaking however there isn't a way to block a single or specific incoming number.  I've known people that have requested their provider to block a certain number and without a very good (usually legal) reason, they won't do it.

The best thing you can do is record 30 seconds of silence in garage band and then use that as a ringtone for that specific number.  Then at least the phone won't alert you.