Can we deny a Site Owner (not Site Col. Admin) access to a Document Library in his site?

Question

We would like to set up a Document Library in a SharePoint Online site with different members, visitors and owners than the Site the Document Library will be created in. It works very well (we have different population for visitors [read] and "members" [edit]) but we can't prevent parent Site Owner's from having ~Full Control access to the Library even if they do not have explicit Full Control permission on (they are not owner of) the Library. Please note: These owners are NOT Sites Collection Administrators (which, we know, cannot be denied access to a library). Thanks, Pierre

Answer 1

Yes, You can deny a Site owner (Not a Site Collection Admin) to have access to one document library. You can achieve this by breaking the permission inheritence for that particular library. go to

Library Settings --> Permission for this document library --> Stop Inheritence permission --> After this select the checkbox Site owner group and click on Remove user permission. and Done

Please note: you can not restrict the site collection admin to access this document library even after performing above steps.

Edit1: As suggested by Derek, you have put the user in a different custom site owner group with a custom permission level where the "Manage Permission" is not available for the person. Know More on SP Permission Level

Answer 2

No. Owners have the "Manage Permissions" permission level, so they can just grant themselves access to secured objects in the site. If you need to secure something away from them, put it in a different site where they do not have those permissions.