You can revert automatic transactions (as these are unauthorized and can be done by parties that know just your account number) but it is impossible to revert iDeal payments.
This isn't true. There is a delay between the iDeal transaction and the money being transferred. If you call your bank immediately, like my mum after being scammed, they can revert it.
It is mandatory to request authorization, but the banks do not have a way to verify that outside of a little flag that says "yes I definitely asked".
https://gocardless.com/guides/sepa/mandates/