Unauthorized debit order

I cancelled my subscription at the end of January and was informed that February would be my final billing period, which I paid.

A debit order was processed again on 1 March, which I dispute as unauthorized.

As this is a prepaid service, I should not be billed for services after cancellation. After cancellation Rain opted to collect the router end of February which they did and yet expect me to pay for the February subscription in March?? Yet their terms n conditions:

Unlimited data (no bundles) No contract (pay monthly, cancel anytime)