I signed up for o/s Roaming. It worked well o/s but it kept auto-renewing the data allowance every week since I returned to NZ (and charged for it). I could not see how to kill the Roaming in MySpark nor the Spark app.
Eventually I rang the Roaming Helpdesk +6433710866 and they were able to stop it. It took them nearly 1/4hr on the 'phone for them to achieve that, apparently struggling with the accounts system. I asked if there was another way to stop the Roaming but the answer was "no".