We are unable to disable sending of Global SMS at the moment. However, sending out of global sms is restricted to maximum of 2 per month.
Ref: Guide for Global SMS & Charges
Maximum number of Global SMS that can be sent without Boosters
A maximum of 2 Global SMS can be sent without the purchase of Global SMS Booster. If there is a need to send more Global SMS, you may purchase Global SMS Boosters by logging in to family.vivifi.me and selecting Manage > VIVIFI Booster.