A bug in the system created a problem there: it sometimes kept the GSM and/or LTE connection active if a user turned the data toggle off while still on Wi-Fi.
According to both Google's own product forums and a helpful post on the Google Fi subreddit, that's no longer the case. Users should be able to manually turn off mobile data and roam free of worry for their wallets. The switch is server-side, so you shouldn't need to do anything to enable it on your Fi-powered phone. But there's another update that will require the latest version of the Google Fi app: call blocking! Another post on the product forum says that Fi version 126.96.36.199 adds the ability to block specific numbers in the Call and Text menu. Users can also access the list on the web at fi.google.com. Blocked numbers won't be able to text or leave voicemail, either.