1. Remotely wipe the phone
2. Ask the carrier to deactivate the SIM
3. Install a new SIM
Meanwhile, Jesse Hollington describes a more convoluted and potentially dangerous process of setting a SIM PIN. Note that you could end up locking yourself out of your phone with this method. Also, it’s limited to Verizon iPhones [1. Actually, this is incorrect. It doesn’t work on Verizon phones. Sorry for the confusion. Thanks to Josh for the correction.] I wouldn’t try it.