How to block people from calling you, with the iPhone still on?

Someone who I'd rather Ignore right now keeps calling me, can I change the settings to not accept calls?

iPhone 4s If It matters

Yes! There's! If you have updated your 4S with IOS 6 it can.

STEPS:

Settings> Do Not Disturb (slide it to ON)

to take it off. Just slide it back to OFF.

there ya go! Hope that helps!