How to block a phone number with a iphone 4s?

I can't figure out how to do it.

Call service provider

You can't.

You can assign a silent ringtone to the numbers you want to block so at least you won't notice them.

In iOS 6, you can't currently block phone calls and text messages. But I have good news! In the new iOS 7 update you can block phone calls, FaceTime calls, and text/iMessages. For now you can just assign them a silent ringtone and wait for the update. Sorry. Hope this helps!