If you can live with the hacking feeling of it, you can set the top margin of your content to -20 and the black stripe will go away. Note that the button on the left will be positioned over your content, which may be a desired behavior, or not.

    <igRibbon:RibbonWindowContentHost>
        <igRibbon:RibbonWindowContentHost.Ribbon>

...

        </igRibbon:RibbonWindowContentHost.Ribbon>
        <DockPanel LastChildFill="True" Margin=0,-20,0,0>

The ribbon group's border is more elaborate than a simple border so that look and feel could not be accomplished with a single border. To change the border thickness/visibility you would need to retemplate it. The default xaml is included in the DefaultStyles directory and you can use this as the basis for your custom template.

Ok, i'm now using TabItem. Is it possible to change the RibbonGroup border (e.g. thicker, none etc) without templating it?

The control is designed to mimic the Office UI Ribbon and to meet the requirements set forth in the Microsoft Office UI Guidelines - the guidelines to which you must adhere to be licensed by Microsoft to be able to use any ribbon ui within your application. Those guidelines require the use of tabs and as such the xamRibbon's template is set up to expect one or more tab items. If this were allowed by the ui guidelines, you would have to retemplate the xamRibbon and determine how you want to layout the elements.

With regards to the Minimize the Ribbon option, you would have to set the AllowMinimize to false.