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Dear Chris,
Thank you for posting in Microsoft Community.
Based on the description, your requirement is to lock the selection so that other user will not be able to book the same date. Please correct me if my understanding is not correct. I found a related thread from Power Automate Community Prevent double booking SharePoint calendar for the... - Power Platform Community (microsoft.com).
Out expert from Power Automate Community found out that the Flow will not trigger when item is Modified only and it was reported to our Product Group. Since there is no out of the box to achieve this, I would suggest you to post the requirement in our Microsoft Power Automate Community. You may also post it in our SharePoint Developer - Microsoft Tech Community.
You kind understanding is highly appreciated. Stay safe and healthy.
Sincerely,
Zee | Microsoft Community Moderator
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