Per my research, we are not able to restrict access to PowerApps by the user in SharePoint. If you do not want users to access PowerApps through the SharePoint site, then it is recommended that you disable PowerApps functionality directly on the page, making it invisible. You could use spfx to upload custom css file to SharePoint online to hide the button.
References to third-party documentation: Disable PowerApps button on list/document library
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