From the Account List page, select your Prepaid account and then click on the "Manage Account" button.
That takes you to your dashboard. Click on the "Deactivate International Roaming" button found under the "I WANT TO" menu.
STEP 4: A pop-up message will then appear on your screen, stating that your request has been received and you will have to wait for an SMS confirming your request status. Click "OK".