As noted in the article, it bans the app from the app stores, not individual devices. It doesn't penalize individuals for having and using the app.

I imagine that they will go after the source. Basically Apple, Google, Samsung and Microsoft will have to hide the app in their respective stores for Montana users. That is, if there isn't an injunction before it even goes anywhere.

The bill states only downloads are banned, so probably just remove it from the App Stores.

Ask Apple to remove it from devices with billing info in that state. Fine Apple until they comply.