Hi Sabine
Unfortunately, it's not possibly to disable this.
From what I was told, Mews will always request a credit card during the online check-in, especially when the reservation wasn't made via the Mews BE. In their way of thinking you can only be sure (or at least very very sure) that it's a valid card of the guest, when the card was entered by the guest and linked to the booking. All cards entered in another way (staff, channel manager, payment request...) are either not linked to the booking (staff or PR) or are not considered as entered by the guest (CM).
This to increase card security for both you and the guests and helping you to prevent chargebacks. Therefore you also have a card on file or any incidentals or damages.
Regards,
Matthias