How do I lock incoming calls?

There are no other options to do that to the phone remotely other than what was described in the previous link:

Report your lost or stolen device to your wireless carrier, so they can disable your account to prevent calls, texts, and data use.

Also note that if you had a passcode on the phone, answering a call does not give them access to data or apps on your phone.

See: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support