MTN automatically signing you up to use airtime for browsing when data is depleted

After the new changes to data and airtime, I bought MTN airtime and converted it to data. When I was done, a message came up saying I have chosen to use airtime when my data is depleted. I did not choose such an option. I also didn't come across such an option. MTN is automatically doing this and there is no way to prominent way to opt out.