The short answer is that smartthings does not provide any level of platform security beyond simple sign in.
There is an FAQ where people talk about various ways of securing their home network, all of which is good information if you’re worried about being hacked, but doesn’t actually solve the issue of someone signing into your SmartThings account.
@tgauchat Is a big proponent of two factor authentication, but is far as I know Samsung has not shown much interest in adding that to smartthings although the option has been suggested for at least four years now.