Is it possible to disable or block firmware updates?

There are several systems that work reliably, it’s just that they don’t do as much as SmartThings does. SmartThings does more, but not as reliably. Different people will make different decisions about that.

BTW, SmartThings is still largely a cloud-based architecture, and even if we did have the ability to postpone firmware updates to the hub, that wouldn’t protect us from the frequent cloud platform changes, most of which are neither announced nor documented.

If the experience is making you crazy, I’d just move on to something else. Every system has pluses and minuses, and it’s a matter of finding the one that matches your own preferences and requirements.