Thank you for writing.
If you want to collapse the whole ribbon please consider the following property;
Alternatively, to collapse a single group, you can use its Visibility
radRibbonBarGroup1.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
If the above mentioned information is not what you are looking for, please provide me with a sketch where the area which you want to collapse is highlighted.
I am looking forward to your reply.
the Telerik team