There are times when blocking all access to mobile data when roaming are desirable and activating the restriction couldn't be easier!
From the dashboard of your my.plan account, go to:
CONTROLS > DATA, CALLS & TEXTS > DATA CONTROLS > ROAMING > [select user] > BLOCKED¹
¹activating the restriction blocks ALL roaming activity but will not prevent roaming activity when connected to a WiFi network. In such cases, no data usage charges will be incurred.