How to stop android using roaming data when it's disabled?

There is a well documented phenomena of Android users getting charged for data roaming abroad when data roaming is explicitly disabled on their phone. `So-called ghost-roaming (Geister-Roaming). In our case, Android 6.0.1 (Samsung S5) we got billed 4 bucks on multiple days each for 60KB (yes, kilobytes) even though data roaming was off. Apparently 4 bucks is the minimum charge per day. The fact that it's always exactly 60 KB shows it's some sort of system/systematic data download ignoring the setting. The network companies refuse to budge because they say it's a bug with the phone. IMO it's a scandal. What can we do to utterly shut down any data roaming?