The code snippets in below examples used to disable or enable the SharePoint Designer access to Site Owners and Site Designers using SharePoint JSOM.

Properties Used: SP.Site.allowDesigner (SP.js)
Supports: SharePoint Online, SharePoint 2010 +

SP.Site.get_allowDesigner() – This property used to get the boolean value, whether the designer access is enabled in Site Collection

SP.Site.set_allowDesigner(true) – This property with true as a parameter used to the enable the SharePoint Designer access to the Site Collection.

SP.Site.set_allowDesigner(false) – This property with false as a parameter used to the disable the SharePoint Designer access to the Site Collection.

Enable SharePoint Designer Access:

Disable SharePoint Designer Access:

This property will not affect the Site Collection Administrators and this property setting only affects the site users.