@Catarina-E-António0, yes, on IB you can cancel penalty free when guest break your house rules. Still, it's never a good idea to do so. So perhaps, before you do that, how about contacting the guest, making it clear that their desired check in time is way outside your published check in time, and see if there is any other solution? Like could he/she book one of your other places, where you have no restriction of check in time in place?
Or, depending on how long they would be staying, perhaps ask them to book that arrival night at an airport hotel, then come over to the booked place the next day, and you'd just change the reservation by shortening it one day? Explore those options.
Or, you could ask them kindly to cancel it themselves, as the new regulation offers guests a penalty free cancelling.