Hi!
With a prepaid card, you can make phone calls, send SMS and go to the Internet, even if no option is booked. Normally, (other provider i.e.) charge 24 cents per megabyte  for the internet, and a simple update to a smartphone could consume all the credit. This has already happened very often.! That's why o2 decided a few years ago to launch Surf Dayflat. The Surf Dayflat is a cost protector and is automatically activated as soon as the mobile phone connects to the Internet without an internet option. The problem with smartphones is that they can also connect to the Internet, even though you have disabled the connection.
You can do the following so that the Surf Dayflat is not booked.
a) Book the smallest internet pack: o2 my Prepaid Basic for 1.99 Euro with infinite data volume (150 MB high-speed volume, then throttled to 32 kBis / s)
b) To block access to the Internet via your o2 Sim card: You can still access the Internet via Wi-Fi. If desired, a moderator can set this lock.
c) Find out if an app or a setting on the phone is responsible for the connection and take action.