There are several brands of door locks supported by SmartThings, all of which could be used to secure someone's front door. After some research, I found it was possible to create a virtual switch to control the lock, so that Google Home can control it.
However, in that thread, there's an interesting point about controlling locks through a voice assistant:
One concern people have raised. If you enable voice control of locks, someone could stand outside your house and tell your voice system to unlock the lock.
Surprisingly (or perhaps not!), this has already happened with an August Smart Lock controlled by Siri:
Last Friday morning, as Marcus was pulling out of the driveway, Mike [Marcus' neighbour] walked up with a grin asking if he could borrow some flour. Marcus responded sure and started to get out of the car to let him in. But before Marcus could do anything, his neighbour said, "I'll let myself in," and ran over to the front door. He then shouted, "Hey Siri, unlock the front door." The door unlocked.
For the setup of a Google Home controlling a SmartThings lock that I proposed at the start of the question, how can I avoid intruders or unwanted visitors from opening my front door? Is the only viable option simply disable it altogether, or might it be possible to modify the setup so that it is more secure?