Thanks for posting in the community.
As per my knowledge, the power automate flow does not have an action to prevent file uploads for now.
You can't change these settings in SharePoint online. You can change the file size upload limit in an on-premises environment, but it's not possible in SharePoint Online because it's a shared setting for all the tenants.
This is a limitation on file uploads in SharePoint Online:
For more limits in SharePoint Online ，please click there: SharePoint Online Limits
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