How to "block" calls?

You have four options.

1. Turn on Do Not Disturb and lock the screen (or configure it to block all calls whether the screen is locked or not on iOS 7)
2. Place the phone in AirPlane mode - this disables the cellular data, SMS and cellular calls as well as WiFi. You can turn on WiFi while in AirPlane mode to access data and still be undisturbed.
3. Use your carrier to start call forwarding. It will send your calls to another number (google voice is a nice place to send them if you don't have another appropriate line). On AT&T, you dial the following to start forwarding, call: **21*1234567890# - assuming your other number is (123)456-7890. To stop the forwarding, call #21#
4. Remove the sim. If you have another sim (same carrier or unlocked phone) that would in effect turn off your primary phone service without deactivating the phone.

Ask you cellular carrier how to forward calls. Then forward the calls to some other number (your office, an answering service, some phone that your cousin never picks up, etc.)