Q: How do I lock my SIM card on an iPhone 5s, to prevent use if stolen/lost?

Contact your carrier and see if they offer this service.

It would be their service, not Apple

You can enter a PIN code if desired. The other option is if the device is stolen, contact the carrier and they can suspend the service on the phone.  Use a SIM PIN for your iPhone or iPad - Apple Support

Generally when phones are stolen, they don't want anything to do with your SIM, it is a way the device can be tracked. They are interested in the device itself.

If you have a passcode lock on the phone, it cannot be used for phone calls, however, the SIM could be removed from the device and used. However, you also have the option to contact your carrier to report the device stolen and they can disable the SIM in that case. Take care.