You can rename the backoffice folder to something else and change the webconfig appSetting to

It wont disable but they wont know the new name. Its good practice to rename this folder anyway.

In Umbraco 10, I was able to accomplish disabling Backoffice by commenting out u.UseBackOfficeEndpoints(); in Startup.cs like so:

app.UseUmbraco()
    .WithMiddleware(u =>
    {
        u.UseBackOffice();
        u.UseWebsite();
    })
    .WithEndpoints(u =>
    {
        u.UseInstallerEndpoints();
        // u.UseBackOfficeEndpoints();
        u.UseWebsiteEndpoints();
    });

Afterwards, the Backoffice URL will result in 404.

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