Folks, can anyone please help me understand and give advice how to resolve this issue?

A man from Lagos (name withheld for now) managed to buy #2000 worth of MTN 'on demand top up' from my Union Bank account.

This fraud was carried out on the 12th and 13th of May - he bought two #1000 top ups for his mobile number 0803 561114; within 24 hours.

I had only just set up an online account 3 days previously. How easy is the MTN top up and how can it be done without some sort of password and/or token?

I have reported to Union Bank and stopped the online facilty but they have not done anything else; no refund, no feedback, no explanation of how it happened. They just said they were investigating... This is maddening as I cannot use my online account and this person is still out there using his mobile and possibly can defraud me again.

Please help - what else can I do?