@Alex107 You can remove a pre-approval if it is within the first 24 hours after you sent it. Here's how:
https://www.airbnb.com/help/article/840/how-do-i-remove-a-pre-approval-that-i-ve-sent-to-a-guest
Your calendar will freed up when you remove it or after the first 24 hours whichever comes first.
A pre-approval expires after 24 hours (and hence you are not able to remove it then) however a guest can still respond to it. When that happens, you will receive an Airbnb message that allows you to decline the pre-approved reservation if you want to.
Hope this helps!