Can DesignPLUS be turned off for a sub-account?

No, DesignPLUS cannot be excluded from any downstream sub-account

Because of the way Canvas loads the themes, we have no way of preventing Canvas from loading the root theme's CSS/JS files in a sub-account.  This means that DesignPLUS styles and sidebar tools, which load from the Canvas theme CSS/JS files, cannot be excluded from any downstream sub-account.